Patent Publication Number: US-2007106767-A1

Title: Database device database search device, and method thereof

Description:
TECHNICAL FIELD  
      The present invention relates to a method of storing data and a structure of data to be stored.  
      More specifically, the present invention relates to a method of storing a series of data associated with one another and a structure of data to be stored.  
     BACKGROUND ART  
      An apparatus called a relational database has been used for storing associated data and for searching for such stored data.  
      Further, for example, Non-Patent Document 1, Patent Documents 1 and 2 disclose methods of storing associated data and methods of searching for such stored data.  
      However, in the relational database, it is not easy to change a structure (schema) of the completed database.  
      In the methods disclosed in Non-Patent Document 1, Patent Documents 1 and 2, descriptive contents of the data become complex, and notation/storage methods for the data are not uniquely determined.  
      [Non-Patent Document 1] 
      The Associative Model of Data White Paper (Lazy Software, September 2000)  
      [Patent Document 1] 
      JP 2001-209647 A  
      [Patent Document 2] 
      WO 00/29980  
     DISCLOSURE OF THE INVENTION  
      [Problems to be Solved by the Invention] 
      The present invention has been made in view of the foregoing background, and an objective of the present invention is to provide a database server in which various kinds of associated data are easily added without changing a structure of a database, a database search device for searching such the database, and methods thereof.  
      Further, the present invention has an objective to provide a database server, in which data is easily described and notation/storage methods for data are unique, a database search device for searching such the database, and methods thereof.  
      [Means for Solving the Problems] 
      To achieve the above-mentioned objectives, the present invention provides a database server for creating a database by associating one or more association nodes with one or more topic nodes, in which each of the association nodes is associated with the one or more topic nodes, and an association attribute is defined to indicate each of associations between the associated association nodes and the topic nodes, and the database server includes: a first entry creation means for creating a first entry including an identifier indicating any of the association nodes, an identifier indicating a topic node associated with the association node, and an association attribute defined between the association node and the topic node; and a first database for storing the one or more of the created first entries.  
      Preferably, the association attribute defined between the associated association node and the topic node indicates a role defined between the nodes.  
      Preferably, for each of the topic nodes, a topic content indicating a content of the topic node is defined, and the database server further includes: a second entry creation means for creating a second entry including an identifier indicating any of the topic nodes, and the topic content defined for the topic node; and a second database for storing the one or more of the created second entries.  
      Preferably, the topic content includes a name of the topic node.  
      Preferably, for each of the association nodes, an association node attribute indicating an attribute of the association node is defined, and the database server further includes: a third entry creation means for creating a third entry including an identifier indicating any of the association nodes, and the association node attribute of the association node; and a third database for storing the one or more of the created third entries.  
      Preferably, for each of the topic nodes, a topic attribute indicating an attribute of the topic node is further defined, and the second entry creation means further creates the second entry including the identifier indicating any of the topic nodes, and the topic content and the topic attribute defined for the topic node.  
      Preferably, the topic node is associated with the one or more association nodes, and when the topic node is associated with plural association nodes, the plural association nodes are associated with one another via the topic node associated with the association nodes, and the second database stores identifiers and topic contents of plural topic nodes associated with the association nodes as a set of plural second entries including the identifiers of the topic nodes.  
      Preferably, the topic node is associated with the one or more association nodes, and when the topic node is associated with plural association nodes, the plural association nodes are associated with one another via the topic node associated with the association nodes, and the second database stores identifiers, topic contents and topic attributes of plural topic nodes associated with the association nodes as a set of plural second entries including the identifiers of the topic nodes.  
      Preferably, the topic node is associated with the one or more association nodes, and when the topic node is associated with plural association nodes, the plural association nodes are associated with one another via the topic node associated with the association nodes, and the first database stores associations between the association nodes and the plural topic nodes associated with the association nodes as a set of plural first entries including the identifiers of the association nodes.  
      Preferably, when there is an association between the two or more association nodes, a new association node is provided, and new topic nodes are provided between the new association node and each of the association nodes, a first association attribute is defined by a relation between the new association node and each of the association nodes and a second association attribute is defined by a relation between the new topic node and the new association node. The first entry creation means creates one or more first entries including the identifier indicating any of the association nodes, an identifier indicating any of the new topic nodes, and the first association attribute, and also creates one or more first entries including an identifier indicating any of the new association nodes, the identifier indicating any of the new topic nodes, and the second association attribute, and the first database stores the created first entries.  
      Preferably, for each of the new topic nodes, a topic content indicating a content of the topic node and a topic attribute indicating an attribute of the topic node are defined, the second entry creation means creates one or more second entries including the new topic node identifier, the topic content, and the topic attribute, and the second database stores the created second entries.  
      Preferably, the third entry creation means creates one or more third entries including the identifier indicating any of the association nodes and its association attribute, and the third database stores the created third entries.  
      Further, the present invention provides a method of creating a database by associating one or more association nodes with one or more topic nodes, in which each of the association nodes is associated with the one or more topic nodes, and an association attribute is defined to indicate each of associations between the associated association nodes and the topic nodes. The database creation method includes: a first entry creation step of creating a first entry including an identifier indicating any of the association nodes, an identifier indicating a topic node associated with the association node, and an association attribute defined between the association node and the topic node; and a first entry storage step of storing the one or more of the created first entries in a first database.  
      Further, the present invention provides a first program for creating a database by associating one or more association nodes with one or more topic nodes, in which each of the association nodes is associated with the one or more topic nodes, and an association attribute is defined to indicate each of associations between the associated association nodes and the topic nodes, and the first program causes a computer to execute: a first entry creation step of creating a first entry including an identifier indicating any of the association nodes, an identifier indicating a topic node associated with the association node, and an association attribute defined between the association node and the topic node; and a first entry storage step of storing the one or more of the created first entries in the first database.  
      Further, the present invention provides a database search device for searching: a first database for storing one or more association nodes, one or more topic nodes respectively associated with each of the association nodes, and an association attribute defined to indicate each of associations between the association nodes and the topic nodes associated together, and for storing one or more first entries each including an identifier indicating any of the association nodes, an identifier indicating a topic node associated with the association node, and an association attribute defined between the association node and the topic node; and a second database for storing one or more second entries including an identifier indicating any of the topic nodes and the topic content defined for the topic node, as data of the topic node, and the database search device includes: search condition receiving means for receiving a search condition including first condition data indicating one or more arbitrary topic contents and one or more arbitrary association attributes, and second condition data indicating one or more second association attributes of the topic nodes as search objects; and search means for searching the first database and the second database based on the received search condition for data of the topic nodes as the search objects.  
      Preferably, the search means performs: (a) a process for searching the second database and creating a topic node identifier set of identifiers of the topic nodes in the second entry including any of the topic contents indicated by the first condition data; (b) a process for searching the first database and creating an association node identifier set of identifiers of the association nodes in the first entry including any one of the topic node identifiers belonging to the topic node identifier set and any one of the association attributes indicated by the first condition data; (c) a process for searching the first database and creating a topic node identifier set of identifiers of the topic nodes in the first entry including any one of the association node identifiers belonging to the association node identifier set and any one of the association attributes indicated by the second condition data; and (d) a process for searching the second database and creating a search result of the topic node as the search object based on the data in the second entry including each of the identifiers belonging to the topic node identifier set.  
      Preferably, the search result obtained in the process (d) includes the topic content of the topic node, and the search means repeats by plural times: (e) a process for searching the second database and creating a topic node identifier set of identifiers of the topic nodes in the second entry including any of the topic contents indicated by the first condition data or the topic contents included in the search result; and the processes (b) to (d). And in the last process (d), the search means creates the search result of the topic node as the search object based on data in the second entry including each of the identifiers belonging to the topic node identifier set.  
      Preferably, for each of the topic nodes, a topic attribute indicating an attribute of the topic node is further defined; the second database stores the second entry including the identifier, the topic content, and the topic attribute; the search condition receiving means receives search conditions including the first condition data, the second condition data, and the third condition data indicating the topic attribute, and the search means searches the first database and the second database for the data of the topic node as the search object based on the received search conditions.  
      Preferably, the search means performs: (f) a process for searching the second database and creating a topic node identifier set of identifiers of the topic nodes in the second entry including any of the topic contents indicated by the first condition data and contained in the second entry, and any of topic attributes indicated by the third condition data and contained in the second entry; (b) a process for searching the first database and creating an association node identifier set of identifiers of the association nodes in the first entry including any one of the topic node identifiers belonging to the topic node identifier set and any one of the association attributes indicated by the first condition data; (c) a process for searching the first database and creating a topic node identifier set of identifiers of the topic nodes in the first entry including any one of the association node identifiers belonging to the association node identifier set and any one of the association attributes indicated by the second condition data; and (d) a process for searching the second database and creating a search result of the topic node as the search object based on the data in the second entry including each of the identifiers belonging to the topic node identifier set.  
      Preferably, the search result obtained in the process (d) includes the topic content and the topic attribute and the topic attribute of the topic node, and the search means repeats by plural times: (g) a process for searching the second database and creating a topic node identifier set of identifiers of the topic nodes in the second entry including any of topic contents indicated by the first condition data or topic contents in the search result contained in the second entry, and topic attributes indicated by the third condition data or topic attributes in the search result contained in the second entry; and the processes (b) to (d). And in the last process (d), the search means creates the search result of the topic node as the search object based on data in the second entry including each of the identifiers belonging to the topic node identifier set.  
      Preferably, in the database search device for further searching a third database storing one or more entries each including the identifier indicating any of the association nodes and an attribute of the association node, the search condition receiving means receives search conditions further including fourth condition data indicating the attribute of the association node, and the search means searches the first database, the second database, and the third database based on the received search conditions for the data of the topic node as the search object.  
      Preferably, in the database search device further including identifier set creation means for creating a set of identifiers of association nodes in each of entries, stored in the third database, including the attributes of the association nodes indicated by the fourth condition in the received search conditions, the search means searches the data of the topic node as the search object using the created association node identifier set.  
      Further, the present invention provides a database search method of searching: a first database for storing one or more association nodes, one or more topic nodes respectively associated with each of the association nodes, and an association attribute defined to indicate each of associations between the association nodes and the topic nodes associated together, and for storing one or more first entries each including an identifier indicating any of the association nodes, an identifier indicating a topic node associated with the association node, and an association attribute defined between the association node and the topic node; and a second database for storing one or more second entry including an identifier indicating any of the topic nodes, and the topic content defined for the topic node, as data of the topic node, and the database search method includes: a search condition receiving step of receiving a search condition including first condition data indicating one or more arbitrary topic contents and one or more arbitrary association attributes, and second condition data indicating one or more second association attributes of the topic nodes as search objects; and a search step of searching the first database and the second database based on the received search condition for data of the topic nodes as the search objects.  
      Further, the present invention provides a second program for searching: a first database for storing one or more association nodes, one or more topic nodes respectively associated with each of the association nodes, and an association attribute defined to indicate each of associations between the association nodes and the topic nodes associated together, and for storing one or more first entries each including an identifier indicating any of the association nodes, an identifier indicating a topic node associated with the association node, and an association attribute defined between the association node and the topic node; and a second database for storing one or more second entries including an identifier indicating any of the topic nodes, as data of the topic node, and the topic content defined for the topic node, the second program causing a computer to execute: a search condition receiving step of receiving a search condition including first condition data indicating one or more arbitrary topic contents and one or more arbitrary association attributes, and second condition data indicating one or more second association attributes of the topic nodes as search objects; and a search step of searching the first database and the second database based on the received search condition for data of the topic nodes as the search objects.  
      [Effect of the Invention] 
      According to the present invention, it is possible to obtain a database server in which various kinds of associated information is easily added without changing a structure of a database, a database search device for searching such a database, and methods thereof.  
      Also, according to the present invention, it is possible to obtain a database server where data is easily described and notation/storage methods for data are unique, a database search device capable of searching such a database, and methods thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram showing a data model when associative network data is converted according to the present invention;  
      FIGS.  2  are diagrams each showing a data structure in which a series of (n) data be stored, which are associated with one another, are converted in accordance with the data model shown in  FIG. 1 ;  
      FIGS.  3  are diagrams each showing an example of a method of expressing an association between associated nodes;  
       FIG. 4  is a diagram showing a structure of associative network data in which data of first and second examples are expressed by using nodes and edges representing associations between the nodes;  
       FIG. 5  is a flowchart of a preferred embodiment of the present invention;  
       FIG. 6  is an exemplification diagram showing a configuration of a database system (DB system) according to the present invention;  
       FIG. 7  is an exemplification diagram illustrating a hardware configuration of the DB server shown in  FIG. 6  and a PC;  
       FIG. 8  is a diagram showing the data associations exemplified in  FIG. 3 ( a ) in a rewritten form;  
       FIG. 9  is a diagram showing the data structure of  FIG. 8  in a generalized form;  
       FIG. 10  is a diagram showing the data associations of FIGS.  3 ( b ) and  4  in a generalized form;  
       FIG. 11  is a diagram showing an association role (AR) table used for storing the data of the structure shown in  FIG. 9 ;  
       FIG. 12  shows a T node identifier (ID) table used for storing the data of the structure shown in  FIG. 9 ;  
       FIG. 13  shows an A node identifier (ID) table used for storing the data of the structure shown in  FIG. 9 ;  
       FIG. 14  is a diagram illustrating a data search method in the DB server shown in  FIGS. 6 and 7 ;  
       FIG. 15  is a first flowchart showing an overall search process (S 20 ) in the DB server shown in  FIGS. 6 and 7 ;  
       FIG. 16  is a flowchart showing an associated node selection process (S 22 ) based on a search filter shown in  FIG. 15 ;  
       FIG. 17  is a flowchart showing a process (S 24 ) of obtaining a node ID and a node name shown in  FIGS. 15 and 16 ;  
       FIG. 18  is a flowchart showing a structure of a DB program  2  run on the DB server shown in  FIGS. 6 and 7 ;  
       FIG. 19  is an exemplification diagram illustrating data input to the DB server (DB program;  FIG. 18 ) shown in  FIGS. 6 and 7 , and search conditions contained therein for searching for data;  
       FIG. 20  illustrates an exemplification AR table created by an AR entry creation unit ( FIG. 18 ) and an ARDB management unit and stored in the ARDB;  
       FIG. 21  illustrates an exemplification T node ID table created by an ID entry creation unit ( FIG. 18 ) and an IDDB management unit and stored in the IDDB;  
       FIG. 22  illustrates an exemplification A node ID table created by the ID entry creation unit ( FIG. 18 ) and the IDDB management unit; and  
       FIG. 23  is an exemplification diagram illustrating a configuration of a second DB system. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
     DETAILED DESCRIPTION OF EMBODIMENTS  
      [Development for Completing the Present Invention] 
      To facilitate understanding of the present invention, a development for completing the present invention will be described first.  
      Data containing various elements are efficiently retrieved, and the data is stored.  
      It is desired that data is read from the stored data when necessary and the data is accurately reproduced.  
      In general, in a partial set R ⊂ A×B of Cartesian product of sets A×B, notation of aRb with respect to an ordered pair (a, b)εR means that “a has a relation R with b”.  
      As a simple example of data, “playwright Shakespeare wrote drama Hamlet” will be taken.  
      The data is in binary relation. Thus, the data “playwright Shakespeare wrote drama Hamlet” is expressed in “aRb” in which a is “playwright Shakespeare”, R is “author-work”, and b is “drama Hamlet” 
      When they are stored in a database, the data is stored as “a”, “R” and “b”, and can be uniquely reproduced when they are read.  
      However, when the number of data components increases, i.e., when the data is not in binary relation but in n-ary relation, a series of such associated data is expressed in a hypergraph structure, and processing of the data is not simple.  
      Accordingly, a method of partitioning the n-ary relation into binary relations, expressing the data in a combination of the binary relations and storing the data in the database is generally employed.  
      As a first example, “playwright Shakespeare wrote drama Hamlet in United Kingdom in about 1600” will be taken.  
      This example shows a quaternary relation. In binary relation, however, the data is expressed in a combination of six binary relations as shown in Table 1 because of  4 C 2 =6.  
                       TABLE 1                       a   R   b                  Playwright Shakespeare   Author - Work   Drama               Hamlet       Playwright Shakespeare   Author - Creation time   About 1600       Playwright Shakespeare   Author - Creation country   England       Drama Hamlet   Work - Creation time   About 1600       Drama Hamlet   Work - Creation country   England       About 1600   Creation time - Creation country   England                  
 
      In other words, when n increases, expression of a binary relation of the number of  n C 2  is necessary to express the n-ary relation.  
      When “playwright Shakespeare wrote drama Twelfth Night” is taken as a second example and expressed in a combination of binary relations, a tertiary relation, i.e., n=3, is set, realizing  3 C 2 =3. A result is as shown in Table 2.  
                       TABLE 2                       a   R   b                  Playwright Shakespeare   Author - Work   Drama Twelfth Night       Playwright Shakespeare   Author - Creation time   About 1600       Drama Twelfth Night   Work - Creation time   About 1600                  
 
      In this case, storage of information of the first example and information of the second example in the same database causes a problem that the same binary relation composed of “playwright Shakespeare”—“author-creation era”—“about 1600” is stored.  
      Another problem that occurs is impossibility of judging what combination of the binary relations shown as the first and second examples should be employed to reproduce original information.  
      Those problems can be solved by adding an identifier to each of the binary relations. However, there is a drawback of complexity in data structure and processing.  
      As a method of storing data having binary or more data components, a storage method using a relational database is known.  
      This method defines/creates a table in which data item names (data attributes) are allocated to columns, and sequentially inputs specific data to lines of the table.  
      Description will be made by taking an example of “playwright Shakespeare wrote drama Hamlet in United Kingdom in about 1600”.  
      (1) “Who”, (2) “what”, (3) “when”, (4) “where”, (5) “why”, and (6) “how” can be specified as data item names.  
      Corresponding to these data items, data of (1) “playwright Shakespeare”, (2) “drama Hamlet”, (3) “about 1600”, (4) “United Kingdom”, (5) “blank”, and (6) “wrote” can be stored.  
      However, the following problems are inherent in the method.  
      (1) It is not easy to add data item names later in accordance with obtained data. Addition is not a problem in the case of data composed of stereotyped components. In the case of inputting data containing various components, however, it is necessary to change a schema for adding data item names each time new components increase. Scheme changing is generally difficult at the time of inputting data on-line.  
      (2) Since the later addition is not easy, during first database construction, a schema may be constructed by listing the maximum number of data items. However, schema construction even for data less likely to be input as data items inevitably leads to a reduction in efficiency of memory use.  
      To solve such problems, a data storage method based on “Associative Model of Data” developed by “Lazy Software” Inc. (England) has been proposed as a model alternative to a conventional relational data model.  
      According to this data model, information is processed in terms of association between objects, and this association is expressed in a “Source-Verb-Target” syntax.  
      This model solves some of the problems inherent in the storage method based on the relational database.  
      However, the model has the following problems. In the case of processing a complex data relation, a method of expressing a relation between data is complex and not intuitive. Expression is not uniquely established because an n-ary relation is expressed in the form of a binary tree. An operator can optionally change the data structure when the data is stored in the database. Thus, in the case of reading and reproducing the data from the database, original information cannot be accurately reproduced.  
      As described above, the relational database has conventionally been known as the method of storing the plurality of data associated with one another.  
      According to this method, data items to be stored are preset, and data relevant to the data items are stored.  
      The method is advantageous in that the data relation can be easily grasped. However, the addition of data items to the constructed database, i.e., the changing of the structure (schema) of the constructed database, is not easy.  
      In the case of the data corresponding to the data items added later, a blank area is generated with respect to the stored data, causing a problem about a reduction in memory use efficiency.  
      On the other hand, in the case of “Associative Model of Data” proposed by the Lazy Software Inc., the problems about the difficulty in adding data items and the reduction in the memory use efficiency are solved. However, the data descriptive contents become complex, and the data structure is not intuitive, causing another problem that the data notation/storage method is not unique.  
      According to the present invention, for associative network data composed of topic nodes and edges representing associations between those nodes, the associations (edges) are newly set as association nodes (referred to as “A nodes” hereinafter in the specification), and a data model composed of edges in which roles (referred to as “association roles” hereinafter in the specification) played by topic nodes associated with the A nodes (referred to as “T nodes” hereinafter in the specification) in the associations are attributes of the edges is defined ( FIG. 1 ).  
      One A node, one T node and an association role therebetween are extracted as basic components from the data structure converted in accordance with the model, and associated with lines (records) of an association role table (referred to as “AR table” hereinafter in the specification) defined in the relational database as shown in Table 3, and a storage/management method is realized by a relational database management system.  
                               TABLE 3                                   A node   T node   Association role                          A1   T1   Association role 1           A1   T2   Association role 2                      
 
      Accordingly, new attribute information regarding certain data (topic node) is defined as another associative data, and expressed as data associated with a line of the AR table by using a combination of corresponding A and T nodes and an edge between the nodes (i.e., basic components in the data model of the present invention), whereby new attribute information can be added without changing the existing table structure (database schema).  
      Furthermore, identifiers are assigned to the A and T nodes to enable unique identification. For the identifiers, an identifier table (referred to as “ID table” hereinafter in the specification) is defined which contains node types representing node attribute types and node names representing attribute values, i.e., specific contents of the nodes (Table 5).  
      The data is stored/managed by the relational database management system as in the case of the AR table.  
      One T node is newly added as data for describing a specific meaning of an association indicated by a certain A node, and these two nodes are associated with each other based on an association role predefined as “reification”.  
      By defining/describing an association between the T node and a T node newly added similarly for another A node, it is possible to express a relation between the associations represented by the two original A nodes.  
      The T node associated based on the association role “reification” particularly introduced to describe the meaning of the A node can be stored/managed by the AR table.  
      By managing the nodes using the ID table and the AR table, a data expression method capable of expressing not only the association between the A and T nodes but also the association between the A nodes is realized.  
      In general, a set of  n C 2  data is necessary when a data set composed of n data having one common association is expressed in binary relation. According to the present invention, however, a set of n data is only necessary.  
      In other words, for a data set composed of n data elements having one common association ( FIG. 2 ( a )), when the data is stored in the database, one new node (A node) common to the data set is added, and then an association role is defined for each element ( FIG. 2 ( b )).  
      Thus, the data structure of the present invention can be constructed by defining the “A node”, the “T node” and the “association role” as one set of data (Table 4).  
                               TABLE 4                                   A node   T node   Association role                          A1   T1   Association role 1           A1   T2   Association role 2           A1   T3   Association role 3           A1   T4   Association role 4           .   .   .           .   .   .           .   .   .           A1   T(n − 1)   Association role (n − 1)           A1   Tn   Association role n                      
 
      “A 1 ” used in  FIG. 2 ( b ) and Table 4 is an identifier assigned to the A node, indicating that data (T nodes) having other identifiers “T 1 ” to “Tn” assigned thereto has a certain common association.  
      If identifiers assigned to the A nodes are different, a series of data have a certain common association in another sense.  
      Furthermore, for the A and T nodes to which the identifiers have been assigned, an ID table having a node type and a node name as data attributes is created as shown in Table 5.  
                               TABLE 5                                   Node ID   Node Type   Node Name                          A1   A Node Type 1   A Node Name 1           T1   T Node Type 1   T Node Name 1           T2   T Node Type 2   T Node Name 2           .   .   .           .   .   .           .   .   .           Tn   T Node Type n   T Node Name n                      
 
      For associative network data in which T nodes T 1  and T 2  are associated with each other by an A node A 1  and T nodes T 1  and T 3  are associated with each other by an A node A 2  ( FIG. 3 ( a )), a T node (identifier T 11 ) is newly added to describe a specific meaning of an association indicated by the A node A 1 , and associated with the A node A 1  based on an association role predefined as “reification”.  
      Similarly, the A node A 2  is associated with a new T node T 12  based on an association role “reification”, and an association between the two T nodes T 11  and T 12  is defined by using the A node A 11  ( FIG. 3 ( b )).  
      Thus, a relation between the two original A nodes A 1  and A 2  can be expressed by using an AR table similar to that shown in Table 6.  
                               TABLE 6                                   A node   T node   Association role                          A1   T11   Reification           A2   T12   Reification           A11   T11   Association role 11           A11   T12   Association role 12                      
 
      [Data Storage Method/Data Structure] 
      Hereinafter, the data storage method and the data structure of the present invention will be described.  
      As a specific example, “playwright Shakespeare wrote drama Hamlet in United Kingdom in about 1600” will be described as first data.  
      The data represents a quaternary relation having elements of (1) “playwright Shakespeare”, (2) “drama Hamlet”, (3) “about 1600”, and (4) “United Kingdom”.  
      When the data is partitioned into binary relations to express a relation thereof, the data is expressed in a combination of six relations as shown in Table 7 because of  4 C 2 =6.  
                       TABLE 7                               Topic       Topic node 1   Link (association)   node 2                  Playwright Shakespeare   Author - Work   Drama               Hamlet       Playwright Shakespeare   Author - Creation time   About 1600       Playwright Shakespeare   Author - Creation country   England       Drama Hamlet   Work - Creation time   About 1600       Drama Hamlet   Work - Creation country   England       About 1600   Creation time - Creation country   England                  
 
      The information is converted according to the present invention. First, the first data is converted.  
      “Playwright Shakespeare” in the topic node  1  indicates “playwright” in this information. Thus, the data is partitioned into “playwright” and “Shakespeare”, the “Shakespeare” is set as a T node, and the “author” is set as an association role. As described later, the “playwright” is set as a node type of a T node.  
      For the edge “author-work” indicating an association, an A node is added for “authorship of Hamlet” so as to indicate that the series of information belong to the same group.  
      The data of “authorship of Hamlet” is used for showing that the series of information belong to the same group. Thus, other expressions are allowed as long as the information can be differentiated from information of the other group.  
      Description in the structure of “A node”—“T node”—“association role” is as shown in Table 8.  
                               TABLE 8                                   A node   T node   Association role                          Authorship of Hamlet   Shakespeare   Author                      
 
      Similarly, “drama Hamlet” in the topic  2  is converted as shown in Table 9 because it is “work” in this information.  
      Additionally, “drama” is set as a node type of a T node.  
                               TABLE 9                                   A node   T node   Association role                          Authorship of Hamlet   Hamlet   Work                      
 
      Next, similar conversion of the second data is as shown in Table 10.  
                               TABLE 10                                   A node   T node   Association role                          Authorship of Hamlet   Shakespeare   Author           Authorship of Hamlet   About 1600   Creation time                      
 
      Here, the data “authorship of Hamlet”, “Shakespeare” and “author” can be omitted because they are redundant data. Thereafter, all the data is similarly converted, redundant data is omitted, and a result is as shown in Table 11.  
                               TABLE 11                                   A node   T node   Association role                          Authorship of Hamlet   Shakespeare   Author           Authorship of Hamlet   Hamlet   Work           Authorship of Hamlet   About 1600   Creation time           Authorship of Hamlet   England   Creation country                      
 
      Accordingly, when a data set composed of four data having one common association is expressed in binary relation, six data sets are necessary because of  4 C 2 =6. However, it can be understood that only four data sets are necessary according to the present invention.  
      In other words, for the data set having four elements, when the data is stored in the database, one new node (A node) common in the data set is added, an association role is then defined for each element, and an “A node”, a “T node” and an “association role” are defined as one set of data. Thus, the data structure of the present invention can be constructed.  
      Here, an identifier “A 1 ” is assigned to the A node “authorship of Hamlet”. It can be understood that the data having common identifiers “A 1 ” belongs to one group.  
      Additionally, identifiers “T 11 ” to “T 14 ” are assigned to four T nodes “Shakespeare”, “Hamlet”, “about 1600” and “United Kingdom”, respectively.  
      Thus, an AR table created from the first example is as shown in Table 12.  
                               TABLE 12                                   A node   T node   Association role                          A1   T11   Author           A1   T12   Work           A1   T13   Creation time           A1   T14   Creation country                      
 
      A node type of the A node (identifier A 1 ) indicating “authorship of Hamlet” is set as “authorship-related information”, and node types of the T nodes to which the identifiers T 11  to T 14  have been assigned are set as “playwright”, “drama”,“era” and “country”. Accordingly, an ID table is created as represented by Table 13.  
                               TABLE 13                                   Node ID   Node Type   Node Name                          A1   Authorship-related information   (NULL)           T11   Playwright   Shakespeare           T12   Drama   Hamlet           T13   Time   About 1600           T14   Country   England                      
 
      As second data, a statement that “Japanese translation with drama Hamlet as original work was published by ∘∘ Publishing Company in February 2003” will be taken as an example.  
      The data is structured as follows according to the present invention. Here, an A node common among a series of data elements is “Japanese translation of Hamlet”, and an identifier “A 2 ” is assigned thereto. A result is as shown in Table 14.  
                               TABLE 14                                   A node   T node   Association role                          A2   Hamlet   Original work           A2   Hamlet   Translation           A2   February 2003   Publication date           A2   ◯◯ Publishing Company   Publication                      
 
      An identifier “T 12 ” has been assigned to Hamlet which is an original work. Thus, when an identifier “T 22 ” is assigned to the translation Hamlet, and identifiers “T 23 ” and “T 24 ” are assigned to the publication date and the publisher, an AR table for the second data is as shown in Table 15.  
                               TABLE 15                                   A node   T node   Association role                          A2   T12   Original work           A2   T22   Translation           A2   T23   Publication date           A2   T24   Publication                      
 
      Additionally, an ID table is created as represented by Table 16.  
                       TABLE 16                       Node ID   Node Type   Node Name                  A2   Authorship-related information   (NULL)       T22   Drama   Hamlet       T23   Date   February 2003       T24   Publisher   ◯◯ Publishing Company                  
 
      The first and second data, and other such data are stored in the same database. Thus, an AR table and an ID table similar to Tables 17 and 18 are eventually obtained.  
                               TABLE 17                                   A node   T node   Association role                          A1   T11   Author           A1   T12   Work           A1   T13   Creation time           A1   T14   Creation country           A2   T12   Original work           A2   T22   Translation           A2   T23   Publication date           A2   T24   Publication           .   .   .           .   .   .           .   .   .                      
 
     
       
         
           
               
               
               
             
               
                 TABLE 18 
               
               
                   
               
               
                   
               
               
                 Node ID 
                 Node type 
                 Node name 
               
               
                   
               
             
            
               
                 A1 
                 Authorship-related information 
                 (NULL) 
               
               
                 T11 
                 Playwright 
                 Shakespeare 
               
               
                 T12 
                 Drama 
                 Hamlet 
               
               
                 T13 
                 Time 
                 About 1600 
               
               
                 T14 
                 Country 
                 England 
               
               
                 A2 
                 Authorship-related information 
                 (NULL) 
               
               
                 T22 
                 Drama 
                 Hamlet 
               
               
                 T23 
                 Date 
                 February 2003 
               
               
                 T24 
                 Publisher 
                 ◯◯ Publishing Company 
               
               
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
               
               
                   
               
            
           
         
       
     
      Furthermore, as shown in  FIG. 4 , T nodes are provided to describe specific meanings of associations indicated by A nodes whose identifiers are A 1  and A 2 . Identifiers “T 31 ” and “T 32 ” are assigned to the T nodes, and the T nodes are associated with the nodes A 1  and A 2  through association roles “reification”.  
      Node types of these two new T nodes are set as “authorship information”, and node names are set as “authorship of Hamlet” and “Japanese translation of Hamlet”, respectively.  
      An A node indicating an association between the nodes T 31  and T 32  is newly added with an identifier A 3 , and a node type is set as “original work-translation information”.  
      Roles of the nodes T 31  and T 32  in the association are “original work information” and “translation information”. Through the aforementioned processing, an AR table and an ID table are added as represented by Tables 19 and 20, respectively.  
                               TABLE 19                                   A node   T node   Association role                          A1   T31   Reification           A2   T32   Reification           A3   T31   Original work information           A3   T32   Translation information                      
 
     
       
         
           
               
               
               
             
               
                 TABLE 20 
               
               
                   
               
               
                   
               
               
                 Node ID 
                 Node type 
                 Node name 
               
               
                   
               
             
            
               
                 T31 
                 Authorship information 
                 Authorship of Hamlet 
               
               
                 T32 
                 Authorship information 
                 Japanese translation of Hamlet 
               
               
                 A3 
                 Original work - Translation 
                 (NULL) 
               
               
                   
                 information 
               
               
                   
               
            
           
         
       
     
      To directly store the first and second data in the relational database, an item (data attribute) corresponding to “Hamlet” as a translation version or “∘∘ Publishing Company” must be newly added as a data item name. This addition is not easy because the table structure of the database is changed.  
      According to the present invention, by adding a line for the existing AR table as shown in the example, it is possible to store data having different data attributes as a plurality of groups.  
      The first data and the second data are as shown in  FIG. 4  when they are expressed by using A nodes, T nodes and edges representing associations between the nodes.  
      The present invention provides a method of easily representing data having such a complex structure and a storage/management method with respect to the relational database.  
      Next, a case of searching for desired data in the database of the present invention will be described.  
      Hereinafter, description will be made by taking as an example a case in which a user “wishes to know publisher of Japanese translation of drama Hamlet written by playwright Shakespeare”.  FIG. 5  is a flowchart showing the process. The flowchart of  FIG. 5  will be described hereinafter.  
       110 : The user inputs search conditions as “drama” written by “playwright”, “Shakespeare”.  
       120 : Plural groups of data satisfying the search conditions are retrieved.  
       130 : Data corresponding to “drama” is sequentially displayed from the detected plural groups of data.  
       140 : The user selects a desired drama name, i.e., “Hamlet”, from the data.  
       150 : Data is searched for with “Hamlet” and “translation” as new search conditions.  
       160 : Plural groups of data satisfying the search conditions are retrieved.  
       170 : Data regarding “publisher” and data regarding “publication date” are sequentially displayed from the detected plural groups of data.  
       180 : The user selects a desired publisher.  
      Detailed description will be given below.  
      The user inputs a node name “Shakespeare” of a T node and an association role “author” as search conditions, and searches for data.  
      An identifier of the T node whose association role is “author” is detected by referring to the AR table in the database. Subsequently, plural groups of identifiers of A nodes corresponding to the T node whose node name is “Shakespeare” are detected by referring to node name attributes stored in the ID table.  
      From the data having the detected plural groups of A node identifiers, another search condition, i.e., a T node identifier whose association role is “work” is selected from the AR table.  
      Data regarding the drama corresponding to the work is sequentially displayed from the ID table based on the identifier.  
      That is, drama names “Hamlet”, “Taming of the Shrew”, “Merchant of Venice”, “Midsummer Night&#39;s Dream”, “King Lear” and the like are sequentially displayed. The user selects a desired drama name, i.e., “Hamlet”.  
      Subsequently, using the identifier “T 12 ” of the selected “Hamlet” as a key, the user searches for an A node identifier containing an identifier of “Hamlet” in a T node ID and having an association role “translation” from the AR table.  
      Accordingly, plural groups of data associated by A node identifiers satisfying the search conditions are detected.  
      Node names of T nodes having “publication” and “publication date” as association roles are sequentially displayed from the detected plural groups of data by referring to the ID table.  
      The user can select a desired publisher therefrom, i.e., “∘∘ Publishing Company” which has published a translation of the drama Hamlet recently in “February 2003”.  
      The example of  FIG. 5  has been described as one search example.  
      Needless to say, when the number of search conditions increases, the number of searching times in the database is not limited to two shown in  FIG. 5 , but repeated by an arbitrary number of times in accordance with desired conditions.  
      In the described specific example, a single attribute is assigned as an association role. However, the association role is not limited to the single attribute.  
      In other words, an association role can have a plurality of attributes. In the example, more detailed association roles can be defined by adding and specifying genres of “tragic drama”, “comedy”, “romance”, “historical drama” and the like to the association role “drama”.  
      [Features of Data Storage Method/Data Structure of the Present Invention] 
      As described above, according to the data storage method and the data structure of an embodiment of the present invention, by using the widely used relational database form, the data can be stored and managed while the hypergraph structure representing a relation of a series of data having tertiary or more complex relations, generally n-ary mutual relations, is maintained.  
      According to the method of directly mapping the associative network data in the table(s) of a relational database in accordance with an embodiment of the present invention, it is possible to solve the prior problem that it is impossible to efficiently store/manage data having tertiary or more complex relations, generally n-ary relations.  
      It is also possible to solve the prior problem that modifications, such as addition of attribute data to the data stored in the database, necessitate changing of the structure of the relational database table, causing a loss of flexibility and requiring much labor.  
      Furthermore, it is possible to give specific meanings to a series of associations by using additional identifiers usually assigned to data and additional association roles in the framework of the same database schema.  
      [Database Server] 
      Hereinafter, a database server of the present invention will be described.  
       FIG. 6  shows a configuration of a database system (DB system)  1  of the present invention.  
      Referring to  FIG. 6 , the first DB system  1  of the present invention is configured by connecting a database server (DB server)  12  to a computer (PC)  102  used for inputting and searching for data via a network  100  such as LAN, WAN or Internet when necessary.  
      In the description below, terms may be partially different from those of the data storage method and the data structure of the present invention described above with reference to  FIGS. 4 and 5  and Tables 7 to 20. However, corresponding terms between the descriptions are substantially identical.  
      In the description of the DB system  1 , terms below will take precedence over the terms of the data storage method and the data structure of the present invention described above.  
      [Hardware Configuration] 
       FIG. 7  shows a hardware configuration of the DB server  12  and the PC  102  shown in  FIG. 6 .  
      Referring to  FIG. 7 , the DB server  12  and the PC  102  include a main unit  120  including a CPU  122 , a memory  124 , and peripheral circuits therefor, input/output devices  126  including a display unit and a keyboard, and a recording device  128  such as a CD or an HDD. Further, when the DB server  12  and the PC  102  (components for performing communication may be generically referred to as communication node, hereinafter) are connected to the network  100 , a communication device  132  for performing communication with the other communication node via the network  100  may be added.  
      In other words, the DB server  12  and the PC  102  include components as computers provided with functions of performing communication with the other communication node.  
      [Data Structure] 
      The DB server  12  is constructed to allow data storage and searching for stored data in accordance with the data storage method and the data structure of an embodiment of the present invention described above with reference to  FIGS. 4 and 5 , and Tables 7 to 20.  
      The data structure and the data search mechanism in the DB server  12  will now be described.  
       FIG. 8  shows the data associations of  FIG. 3 ( a ) in a rearranged form.  
      Referring to  FIG. 8 , in the DB server  12 , a topic node (T node, hereinafter) is associated with one or more association nodes (A nodes, hereinafter), and an association attribute R is defined between the associated T and A nodes.  
      The association attribute R may be any attribute for defining an association between the T and A nodes. However, for a specific and clear description presented below, a specific example in which the association attribute R is an association role R (as detailed in the aforementioned description of the data storage method and the data structure of an embodiment of the present invention) will be considered.  
      The data associations shown in  FIG. 3 ( a ) can be rearranged as shown in  FIG. 8 .  
      Among A nodes A 1  to An shown in  FIG. 8  (n is an integer of 1 or more, but n does not indicate the same number in all cases), the A node A 1  and T nodes T 1 - 1  to T 1 - 3 , and T 2 - 1  associated with the A node A 1  are interconnected by links.  
      Similarly, T nodes T 2 - 1 , T 2 - 2  and Tn- 1  associated with the A node A 2 , and the A node A 2  are interconnected by links.  
      The same holds true for the A node An. T nodes Tn- 1  to Tn- 4  associated with the A node An, and the A node An are interconnected by links.  
      In other words,  FIG. 8  shows that the T node T 2 - 1  has associations with both of the A nodes A 1  and A 2 , and the T node Tn- 1  has associations with both of the A nodes A 2  and An.  
       FIG. 9  shows the data structure of  FIG. 8  in a generalized form.  
      The links  851 ,  852 ,  853 ,  854 ,  855  from the T node T 1 - 1  through the A node A 1 , the T node T 2 - 1 , the A node A 2  and the T node Tn- 1  to the A node An in  FIG. 8  are represented along a path  961  extended in a top-to-bottom direction in  FIG. 9 .  
      In a specific example,  FIG. 9  additionally shows the following.  
      (1) The T nodes T 1 - 1  to T 1 -m 1  and T 2 - 1  are associated with the A node A 1 , and association roles R 1 - 1  to R 1 -m 1  and R 1 - 0  are defined in associations (links) between the T nodes T 1 - 1  to T 1 -m 1  and T 2 - 1  and the A node A 1 .  
      (2) The T nodes T 2 - 1  to T 2 -m 2  and the T node omitted from  FIG. 9  are associated with the A node A 2 , and association roles R 2 - 1  to R 2 -m 2  are defined in associations between the T nodes T 2 - 1  to T 2 -m 2  and A node A 2 .  
      (3) Similarly thereafter, the T node omitted from  FIG. 9  is associated with the A node, and an association role R is defined therebetween.  
      (4) The T nodes Tn- 1  to Tn-mn and the T node omitted from  FIG. 9  are associated with the A node An, and association roles Rn- 1  to Rn-mn (m 1  to mn, and n is an integer) are defined in associations between the T nodes Tn- 1  to Tn-mn and the A node An.  
      In other words, in the DB server  12 , each of the T nodes is associated with one or more A nodes, and each of the A nodes is associated with one or more T nodes, whereby the plurality of T nodes can be associated with one another via the A nodes, and the plurality of A nodes can be associated with one another via the T nodes.  
      In the DB server  12 , a plurality of combinations of the A and T nodes associated as shown in  FIG. 9  can be stored.  
       FIG. 10  is a diagram showing the data associations of FIGS.  3 ( b) and  4  in a generalized form.    
      Referring to  FIG. 10 , the T nodes T 1 - 1  to T 1 - 3  (, and T 3 - 1 ) associated with the A node A 1  are connected to the A node A 1  by links, the T nodes T 2 - 1  to T 2 - 3  (, and T 3 - 2 ) associated with the A node A 2  are connected to the A node A 2  by links, and the A node An and the T nodes Tn- 1  to Tn- 3 , and T 2 - 3  (, and T 3 -n) are connected together by links.  
      The T node T 2 - 3  is connected to both of the A nodes A 2  and An by links, which means that the T node T 2 - 3  is associated with both of the A nodes A 2  and An.  
      When, like in this case, a series of information associated by the A nodes A 1 , A 2  and An have a common association, a new association node A 3  can be defined.  
      For example, when the A node A 1  is information regarding the original work of Hamlet, the A node A 2  is information regarding the translation of Hamlet, and the A node An is information regarding the performance of Hamlet, associative data represented by the A nodes A 1 , A 2  and An have a commonality as data regarding Hamlet.  
      Thus, to indicate that the data associated by the A nodes A 1 , A 2  and An have the commonality, the new association node A 3  is defined and stored in the database.  
      As shown by broken line in  FIG. 10 , to specifically describe the series of information represented by the A node A 1  and the T nodes T 1 - 1  to T 1 - 3 , a new T node T 3 - 1  is defined and stored in the database.  
      Similarly, to specifically describe the associations of the A nodes A 2  and An, new T nodes T 3 - 2  and T 3 -n are defined and stored in the database.  
      For example, data “authorship of Hamlet” is defined as topic contents in the T node T 3 - 1 , data “Japanese translation of Hamlet” is defined as topic contents in the T node T 3 - 2 , and data “performance of Hamlet” is defined as topic contents in the T node T 3 -n. These data are stored in the database.  
      Additionally, an association role R is defined between the new A node A 3  and each of the T nodes T 3 - 1  to T 3 -n, and stored in the database.  
      For example, “original work information” is defined as an association role R between the new A node A 3  and the T node T 3 - 1 , “translation information” is defined as an association role R between the new A node A 3  and the T node T 3 - 2 , and “performance information” is defined as an association role R between the new A node A 3  and the T node T 3 -n. These data are stored in the database.  
      Similarly, for example, association roles R predefined as “reification” by the system are defined between the A nodes A 1 , A 2  and An and the T nodes T 3 - 1 , T 3 - 2  and T 3 -n.  
       FIG. 11  shows an association role (AR) table used for storing the data of the structure shown in  FIG. 9 .  
       FIG. 12  shows a T node identifier (ID) table used for storing the data of the structure shown in  FIG. 9 .  
       FIG. 13  shows an A node identifier (ID) table used for storing the data of the structure shown in  FIG. 9 .  
      In the DB server  12 , the data of the A and T nodes associated by the structure shown in  FIG. 9  and the data of the T node are stored by using the AR table of  FIG. 11  and the ID table of  FIG. 12 .  
      Each of the entries (or records or rows) of the AR table shown in  FIG. 11  represents one given A node, one T node associated with the A node, and an association role R defined between the associated A and T nodes, and includes the A node ID, the T node ID, and the association role R.  
      In other words, each of the entries of the AR table includes the identifier of the A node at one end of one of the links shown in  FIG. 9 , the identifier of the T node at the other end of the link, and the association role defining an attribute of the link.  
      Such entries are created for all the links (links between the T node T 1 - 1  and the A node A 1  and between the T node Tn-mn and the A node An) shown in  FIG. 9 , and stored in the AR table. Accordingly, the associations between the A and T nodes shown in  FIG. 9  are stored in the AR table of  FIG. 11 .  
      Each T node has its contents (name of the T node, data of the T node itself, data referred to by the T node, and the like). Further, for each T node, in addition to the identifier (ID) stored in each entry of the AR table, an attribute of the T node (node type (NT); topic attribute) is defined. Hereinafter, a specific example in which each T node has only its name (node name (N)) as its contents will be considered.  
      Each entry of the T node ID table shown in  FIG. 12  includes an identifier (ID) of one of the T nodes shown in  FIG. 9 , an attribute (node type (NT)) defined for the T node, and a name (node name (N)) of the T node.  
      Such entries are created for all the T nodes T 1 - 1  to Tn-mn shown in  FIG. 9 , and stored in the T node ID table. Accordingly, the data of all the T nodes shown in  FIG. 9  are stored.  
      Each entry of the A node ID table shown in  FIG. 13  includes an identifier (ID) of one of the A nodes shown in  FIG. 9 , an attribute (node type (NT′)) defined for the A node, and a name (node name (N′)) of the A node.  
      Such entries are created for all the A nodes A 1  to An shown in  FIG. 9 , and stored in the A node ID table. Accordingly, the data of all the A nodes shown in  FIG. 9  are stored.  
      It is to be noted that similar forms can be employed to store the associations between the A and T nodes and the data of the A and T nodes shown in  FIG. 9  in addition or as alternative to the table forms. In the description below, however, a specific example of using the AR table and the ID tables will be considered.  
      Depending on use, configuration or processing task of the DB server  12 , as shown in  FIG. 11 , each entry may include contents (node name (N)) of the T node in place of the identifier (ID) of the T node in the AR table.  
      Similarly, in the AR table, each entry may further include contents of the T node.  
      [Data Search] FIG. 14  shows a data search method in the DB server  12  shown in  FIGS. 6 and 7 .  
      As shown in  FIG. 14 , a specific example will be considered in which T nodes T 1  to Tn and a T node Tret (T return), a search-result-to-be(output), are associated with a certain A node, association roles R 1  to Rn and Rret are defined between the A node and the T nodes T 1  to Tn and Tret, and the T nodes T 1  to Tn and Tret have node names N 1  to Nn and Nret.  
      In the DB server  12 , one or more combinations of the association role Rret defined for Tret, an attribute of the A node used for searching (node type NT; ANT 1  and ANT 2  in  FIG. 14 ), the association role R and the node name N of the T node used for the searching are used as search conditions.  
      As shown in  FIG. 14 , for example, the search conditions are represented by (Rret, (ANT 1 , ANT 2 , . . . ), Filter), where Filter=((R 1 , N 1 ), (R 2 , N 2 ), . . . , (Rn, Nn)).  
      The search conditions can further contain an attribute NT of the T node (third condition data) as described later.  
      Among the search conditions, one or more combinations (R 1 , N 1 ), (R 2 , N 2 ), . . . (Rn, Nn) of the association role R and the node name N of the T node included in the Filter are used as filters for searching, and thus will be referred to as search filters hereinafter.  
      Among the search conditions, the attributes of the A node (ANT 1 , ANT 2 , . . . ) can be omitted.  
       FIG. 15  is a first flowchart showing an overall process (S 20 ) of the searching process in the DB server  12  shown in  FIGS. 6 and 7 .  
      Referring to  FIG. 15 , in a step S 200 , for example, the DB server  12  accepts the search conditions shown in  FIG. 14  which has been entered, e.g., by the searcher using the input/output device  126  of the PC  102  ( FIG. 6 ) or the DB server  12 .  
      In a step S 22 , detailed in  FIG. 16 , an association node is selected based on a search filter which will be described later.  
      In a step S 24 , detailed in  FIG. 17 , a node ID and a node name which will be described later are obtained.  
      In a step S 202 , the DB server  12  creates a response to the searcher&#39;s query based on an identifier (node ID) and the node name (Nret) of the T node Tret obtained as a search result through the processing in S 24 .  
      For the response, only the node name Nret, various data referred to by the node Tret, or various data indicating the node Tret can be used.  
      In a step S 204 , referring to  FIG. 15 , the DB server  12  determines whether searcher&#39;s query has been terminated or not.  
      The DB server  12  ends the process when the searcher&#39;s query has been terminated, or returns to the processing in S 200  otherwise.  
       FIG. 16  is a flowchart showing an association node selection process (S 22 ) based on the search filter shown in  FIG. 15 .  
      Referring to  FIG. 16 , upon the acceptance of the search conditions (Rret, (ANT 1 , ANT 2 , . . . ), Filter), Filter=((R 1 , N 1 ), (R 2 , N 2 ), . . . , (Rn, Nn)) in the processing in S 200  shown in  FIG. 15 , the DB server  12  initializes a list of association nodes used for processing in a step S 220 .  
      In this association node list, among the A nodes obtained from the AR table ( FIG. 11 ), an identifier of an A node containing one of the attributes of the A nodes (node type; ANT 1 , ANT 2 , . . . ) in the search conditions as its attribute (node type NT) is stored.  
      It is to be noted that when the attributes of the A nodes are omitted from the search conditions ((ANT 1 , ANT 2 , . . . )=NULL), in the processing in S 220 , all the identifiers of the A nodes obtained from the AR table ( FIG. 11 ) are stored.  
      In a step S 222 , the DB server  12  determines whether processing has been carried out or not for all the search filters (Ri, Ni).  
      The DB server  12  proceeds to processing in S 24  ( FIGS. 15 and 17 ) if the processing has been carried out for all the search filters. Otherwise, the DB server  12  sets any one of the search filters (Ri, Ni) not yet processed as the next filter for processing, and proceeds to processing in S 224 .  
      In step S 224 , the DB server  12  searches the T node ID table ( FIG. 12 ) to retrieve all entries containing the node names Ni of the search filters (Ri, Ni), and creates a set of T node identifiers contained in the retrieved entries (node ID set T, where T={Ti|node name=Ni in ID table}).  
      It is to be noted that when the search conditions contain attributes of T nodes (node type; NT) and the search filters are represented by (Ri, Ni, NTi), in the processing in S 224 , the DB server  12  only needs to retrieve entries containing the node names Ni and the node types NTi of the search filters (Ri, Ni, NTi) from the ID table, and to create a set of T node identifiers contained in the retrieved entries as a node ID set T.  
      In a step S 226 , the DB server  12  determines whether the node ID set T obtained in the processing in S 224  is empty or not.  
      When the node ID set T is empty, the DB server  12  performs processing for terminating the search process (display a “zero matches found” message or the like to the searcher), and ends the search process. Otherwise, the DB server  12  proceeds to processing in S 228 .  
      In step S 228 , the DB server  12  searches the AR table ( FIG. 11 ) to update the association node list A.  
      That is, the DB server  12  retrieves all the entries containing the association roles Ri of the search filters (Ri, Ni) and the T node identifiers included in the node ID set T obtained in the processing in S 224  from the AR table, and stores the A node identifiers contained in the retrieved entries in the association node list A (A={Aj|role=Ri, T node identifier=Ti (all i), A node identifier εA in AR table}).  
      In a step S 230 , the DB server  12  determines whether the association node list A obtained in the processing in S 228  is empty or not.  
      When the association node list A is empty, the DB server  12  performs processing for terminating the search process, and ends the search process. Otherwise, the DB server  12  proceeds to processing in S 232 .  
      In step S 232 , the DB server  12  reads the search filter included in the search conditions but not processed and returns to the processing in S 222 .  
       FIG. 17  is a flowchart showing a node ID and node name obtaining process S 24  shown in  FIGS. 15 and 16 .  
      Referring to  FIG. 17 , upon the completion of the association node selection process S 22  based on the search filters, in a step S 240 , the DB server  12  searches the AR table ( FIG. 11 ) to create a T node ID set T.  
      That is, the DB server  12  retrieves all the entries containing the association roles Rret included in the search conditions and A node identifiers included in the association node list A obtained in the processing in S 22  (S 228 ) from the AR table, and creates a set of T node identifiers (T node ID set T) contained in the retrieved entries (T={Tm|role=Rret, A node identifier εA in AR table}).  
      In a step S 242 , the DB server  12  determines whether the T node ID set T obtained in the processing in S 240  is empty or not.  
      When the T node ID set T is empty, the DB server  12  performs termination processing to end the search process. Otherwise, the DB server  12  proceeds to S 244 .  
      In step S 244 , the DB server  12  searches the T node ID table ( FIG. 12 ) to create a node ID and node name set P.  
      That is, the DB server  12  retrieves from the ID table all the entries containing any of the T node identifiers Tm contained in the T node ID set T created in the processing in S 240 , and creates a set P of node names Nm and T node identifiers Tm contained in the entries (P={(Tm, Nm)| T node identifier=Tm (all m) in ID table}).  
      In a step S 246 , the DB server  12  determines whether the set P of the T node identifiers and the T node names obtained in the processing in S 244  is empty or not.  
      The DB server  12  performs termination processing to end the search process when the set P of the T node identifiers and the T node names is empty. Otherwise, the DB server  12  proceeds to processing in S 202 .  
      This set P is used for creating the response to the searcher in the processing in S 202  shown in  FIG. 15 .  
      [DB Program  2 ] 
       FIG. 18  shows a structure of a DB program  2  executed in the DB server  12  shown in  FIGS. 6 and 7 .  
      In  FIG. 18 , for clear illustration purposes, lines showing the data flow are omitted if not required.  
      Referring to  FIG. 18 , the DB program  2  includes a DB management unit  20 , a DB unit  24  and a DB search unit  26 .  
      The DB management unit  20  includes a management operation receiver  200 , an AR entry creation unit  202 , an ID entry creation unit  204 , an AR database management unit (ARDB management unit)  206 , and an ID database management unit (IDDB management unit)  208 .  
      The DB unit  24  includes an AR database (ARDB)  240 , a T node ID database (IDDB)  242 , and an A node IDDB  244 .  
      The DB search unit  26  includes a search operation receiver  260 , a search condition creation unit  262 , a search control unit  264 , an AR database search unit (ARDB search unit)  266 , and an ID database search unit (IDDB search unit)  268 .  
      For example, the DB program  2  is carried on and read from the recording medium  130  ( FIG. 7 ) to the DB server  12 , loaded into the memory  124 , and executed under an operating system in the DB server  12  by specifically using hardware of the DB server  12  (similar for each program below).  
      With these components, the DB program  2  is used to create an AR database ( FIG. 11 ) and an ID database ( FIGS. 12 and 13 ) described above with reference to FIGS.  9  to  13 , and perform data search using the databases (FIGS.  14  to  17 ).  
      In the DB unit  24 , the ARDB  240  stores the AR table shown in  FIG. 11 .  
      The IDDB  242  stores the T node ID table shown in  FIG. 12 .  
      The IDDB  244  stores the A node ID table shown in  FIG. 13 .  
       FIG. 18  shows a specific example in which the T node ID table and the A node ID table shown in  FIGS. 12 and 13  are stored in the IDDBs  242  and  244 . However, the T node ID table and the A node ID table may be stored in the same database.  
      The T node ID table and the A node ID table do not need to be always created separately, but they may be integrally created in one database.  
      In the DB management unit  20 , the management operation receiver  200  receives an operation of managing or modifying data stored in the AR table and the ID table from the input/output device  126  ( FIG. 7 ) or from the PC  102  ( FIG. 6 ) through the network  100 , and outputs the operation to the ARDB management unit  206  and the IDDB management unit  208 .  
      The management operation receiver  200  receives user&#39;s operation for designating an A node and a T node, an association between the A node and the T node, an association role R defined between the A node and the T node (links), identifies (ID) assigned to the A node and the T node, a node name (N) assigned to the T node, and an attribute ( FIG. 9 ) defined for the T node, and outputs the operation to the AR entry creation unit  202  and the ID entry creation unit  204 .  
      For example, the management operation receiver  200  displays a user interface (UI) image representing the A node and the T node, an association therebetween as shown in  FIG. 14  in the input/output device  126 , receives user&#39;s operation on the UI image, and accepts the designations.  
      The AR entry creation unit  202  creates entries of the AR table shown in  FIG. 11  according to user&#39;s designations input from the management operation receiver  200 , and outputs the entries to the ARDB management unit  206 .  
      The ARDB management unit  206  adds the entries of the AR table input from the AR entry creation unit  202  to the AR table stored in the ARDB  240 .  
      The ARDB management unit  206  modifies contents of the AR table stored in the ARDB  240  according to user&#39;s operation input from the management operation receiver  200 .  
      The ARDB management unit  206  retrieves the entries of the AR table stored in the ARDB  240  according to search requests by the ARDB search unit, and outputs the entries to the ARDB search unit  266 .  
      The ID entry creation unit  204  creates entries for the T node and A node ID tables shown in  FIGS. 12 and 13  according to user&#39;s designation input from the management operation receiver  200 , and outputs the entries to the IDDB management unit  208 .  
      The IDDB management unit  208  adds the entries of the T node ID table input from the ID entry creation unit  204  to the T node ID table stored in the IDDB  242 .  
      The IDDB management unit  208  adds the entries of the A node ID table input from the ID entry creation unit  204  to the A node ID table stored in the IDDB  244 .  
      The IDDB management unit  208  modifies contents of the ID table stored in the IDDBs  242  and  244  according to user&#39;s operation input from the management operation receiver  200 .  
      The IDDB management unit  208  retrieves the entries of the ID table stored in the IDDBs  242  and  244  according to search requests by the IDDB search unit  268 , and outputs the entries to the IDDB search unit  268 .  
      In the DB search unit  26 , the search operation receiver  260  receives searcher&#39;s operation for designating search conditions ( FIG. 14 , attribute of an optional T node (node type (NT)) may be further contained) used for the search process shown in FIGS.  14  to  17  from the input/output device  126  ( FIG. 7 ) or from the PC  102  ( FIG. 6 ) through the network  100 .  
      The search operation receiver  260  outputs the received operation to the search condition creation unit  262 .  
      For example, when the search operation receiver  260  accepts search conditions in a form of a natural language query, the search condition creation unit  262  parses the query statement to take out words.  
      Next, the search condition creation unit  262  searches the AR table and the ID table stored in the ARDB  240  and the IDDBs  242  and  244  through the ARDB search unit  266 , the IDDB search unit  268 , the ARDB management unit  206  and the IDDB management unit  208 , and extracts words used as search conditions.  
      Further, the search condition creation unit  262  combines the extracted words according to the structure of the query sentence, retrieves the search conditions in the form of (Rret, (ANT 1 , ANT 2 , . . . ), ((R 1 , N 1 ), (R 2 , N 2 ), . . . , (Rn, Nn)) shown in  FIG. 14 , and outputs the search conditions to the search control unit  264 .  
      It is to be noted that when the searcher directly designates the search conditions in the form of (Rret, (ANT 1 , ANT 2 , . . . ), ((R 1 , N 1 )), (R 2 , N 2 ), . . . , (Rn, Nn))) shown in  FIG. 14 , the search condition creation unit  262  may be omitted.  
      The search condition creation unit  262  may be a tool for assisting retrieval of the search conditions (Rret, (ANT 1 , ANT 2 , . . . ), ((R 1 , N 1 ), (R 2 , N 2 ), . . . , (Rn, Nn))) by the searcher.  
      The search control unit  264  controls the ARDB search unit  266  and the IDDB search unit  268  according to the search conditions (Rret, (ANT 1 , ANT 2 , . . . ), ((R 1 , N 1 ), (R 2 , N 2 ), . . . , (Rn, Nn))) input from the search condition creation unit  262  (search operation receiver  260 ) to perform searching in the ARDB  240  (AR table;  FIG. 11 ) and the IDDBs  242 ,  244  (ID tables;  FIGS. 12 and 13 ) through the ARDB management unit  206  and the IDDB search unit  208  as shown in FIGS.  15  to  17 .  
      When a search result (set P;  FIG. 17 ) is obtained by the searching based on the search conditions, the search control unit  264  creates a response based on the search result, displays the response in the input/output device  126  ( FIG. 7 ), or displays the response in the input/output device  126  of the PC  102  through the network  100  ( FIG. 6 ) to the searcher.  
      The ARDB search unit  266  searches the ARDB  240  (AR table;  FIG. 11 ) through the ARDB management unit  206  under the control of the search control unit  264 , and returns the search result to the search control unit  264 .  
      The IDDB search unit  268  searches the IDDBs  242  and  244  (ID tables;  FIGS. 12 and 13 ) through the IDDB management unit  208  under the control of the search control unit  264 , and returns the search result to the search control unit  264 .  
      [Overall Operation] 
      Hereinafter, an overall operation of the DB server  12  (DB program  2 ;  FIG. 18 ) shown in  FIGS. 6 and 7  will be described with respect to specific examples.  
      [Creation of AR Table and ID Table] 
      To begin with, a process of creating an AR table and ID tables by the DB management unit  20  of the DB program  2  will be described.  
       FIG. 19  shows data input to the DB server  12  (DB program  2 ;  FIG. 18 ) shown in  FIGS. 6 and 7 , and search conditions to perform searching for data contained therein.  
      For example, data associated as shown in  FIG. 19  are input to the management operation receiver  200  of the DB program  2 .  
      The data shown in  FIG. 19  contain A nodes and T nodes associated as described below, and node names are assigned to the T nodes (however, attributes of the A nodes (node type (NT) and node names, and attributes of the T nodes (node type (NT)) are omitted from subsections (1) to (8) below and  FIG. 19 ).  
      (1) “A node A 1 ” and “T node T 11 ” are associated together, an association role R “Author” is defined therebetween, and a node name “Shakespeare (different person of the same name)” is assigned to the “T node T 11 ”.  
      (2) “A node A 9 ” and “T nodes T 92  and T 41 ” are associated together, association roles R “Work” and “Author” are defined therebetween, and node names “The Merchant of Venice” and “Shakespeare” are assigned to the “T nodes T 92  and T 42 ”.  
      (3) “A node A 4 ” and “T nodes T 41  and T 42 ” are associated together, association roles R “Author” and “Work” are defined therebetween, and a node name “Hamlet” is assigned to the “T node T 42 ”.  
      (4) “A node A 13 ” and “T node T 42 ” are associated together, and an association role R “Script” is defined therebetween.  
      (5) “A node A 19 ” and “T node T 42 ” are associated together, and an association role R “Original Work” is defined therebetween.  
      (6) “A node A 10 ” and “T node T 42 ” are associated together, and an association role R “Original Work” is defined therebetween.  
      (7) “A node A 10 ” and “T nodes T 103  and T 101 ” are associated together, association roles R “Publication” and “Translation” are defined therebetween, and a node name “∘∘ Publishing Company” is assigned to the “T node T 103 ”.  
      (8) “A node A 19 ” and “T node T 191 ” are associated together, and an association role R “Translation” is defined therebetween.  
      The management operation receiver  200  receives input data, and outputs the data to the AR entry creation unit  202  and the ID entry creation unit  204 .  
      The AR entry creation unit  202  creates each entry of the AR table from the data shown in  FIG. 19 , and outputs the entry to the ARDB management unit  206 .  
       FIG. 20  shows an AR table created by the AR entry creation unit  202  ( FIG. 18 ) and the ARDB management unit  206  and stored in the ARDB  240 .  
      It is to be noted that in the drawings below, NULL indicates that there is no attribute (node type)/name (node name).  
      The ARDB management unit  206  sequentially adds the entries of the AR table input from the AR entry creation unit  202  to the AR table stored in the ARDB  240 .  
      As a result of the processing by the AR entry creation unit  202  and the ARDB management unit  206 , an AR table shown in  FIG. 20  is created from data shown in  FIG. 19 , and stored in the ARDB  240 .  
       FIG. 21  shows a T node ID table created by the ID entry creation unit  204  ( FIG. 18 ) and the IDDB management unit  208  and stored in the IDDB  242 .  
      The ID entry creation unit  204  creates entries of the T node ID table from the data shown in  FIG. 19 , and outputs the entries to the IDDB management unit  208 .  
      The IDDB management unit  208  sequentially adds the entries of the T node ID table input from the ID entry creation unit  204  to the ID table stored in the IDDB  242 .  
      As a result of the processing by the ID entry creation unit  204  and the IDDB management unit  208 , a T node ID table shown in  FIG. 21  is created from the data shown in  FIG. 19 , and stored in the IDDB  242 .  
       FIG. 22  shows an A node ID table created by the ID entry creation unit  204  ( FIG. 18 ) and the IDDB management unit  208 .  
      The ID entry creation unit  204  creates entries of the A node ID table from the data shown in  FIG. 19 , and outputs the entries to the IDDB management unit  208 .  
      The IDDB management unit  208  sequentially adds the entries of the ID table input from the ID entry creation unit  204  to the A node IDDB table stored in the IDDB  244 .  
      As a result of the processing by the ID entry creation unit  204  and the IDDB management unit  208 , an A node ID table similar to that shown in  FIG. 22  is created from the data shown in  FIG. 19 , and stored in the IDDB  244 .  
      [Data Search] 
      For example, when the searcher inputs a search condition “what is the name of the publisher that publishes a translation of the original work of drama Hamlet which is one of the works that the Playwright Shakespeare wrote as an author?” in a form of a query statement to the input/output device  126  ( FIG. 7 ) of the DB server  12 , the search condition creation unit  262  analyzes this query statement, and parses the query statement into two parts, first and second halves as shown in  FIG. 19 .  
      In the second half of the query statement “the works that the Playwright Shakespeare wrote as an author”, to retrieve data regarding “Work”, the search condition creating unit  262  sets an association role Rret of the T node Tret to be a search result as “Work”.  
      The search condition creation unit  262  creates a search filter (R 1 =“Author”, N 1 =“Shakespeare”) from the second half “Shakespeare wrote as an author”.  
      Further, the search condition creation unit  262  creates search conditions (Rret, (ANT 1 ), ((R 1 , N 1 )))=(Work, (NULL), ((Author, Shakespeare))) corresponding to the second half of the query statement from the association role (Rret=“Work”) of the T node Tret and the search filter (R 1 =“Author”, N 1 =“Shakespeare”).  
      In the first half of the query statement “what is the name of the publisher that publishes a translation of the original work of drama Hamlet?”, to retrieve information regarding “Publisher that publishes”, the search condition creating unit  262  sets an association role Rret of the T node Tret to be a search result as “Publication”.  
      The search condition creation unit  262  creates a first search filter (R 1  =“Original Work”, N 1 =“Hamlet”) from the condition that an “original work” of drama “Hamlet” included in the first half, and creates a second search filter (R 2 =“Translation”, N 2 =“NULL (not specified)”) from “Translation” included in the first half.  
      Further, the search condition creation unit  262  creates search conditions (Rret, (ANT 1 ), ((R 1 , N 1 ), (R 2 , N 2 )))=(Publication, (NULL), ((Original Work, Hamlet), (Translation, NULL))) corresponding to the first half of the query statement from the association role (Rret=“Publication”), the first search filter (“R 1 =“Original Work”, N 1 =“Hamlet”) and the second filter (R 2 =“Translation”, N 2 =“NULL”).  
      Based on the search conditions created by the search condition creation unit  262 , the search control unit  264  searches the ARDB  240  (AR table;  FIG. 20 ) and the IDDB  242  (T node ID table;  FIG. 21 ) through the ARDB search unit  266  and the IDDB search unit  268  to obtain search results as described below.  
      First, from the search conditions obtained from the second half of the query statement, the search control unit  264  performs the following.  
      (1) The search control unit  264  refers to the T node ID table stored in the IDDB  242  ( FIG. 18 ) to retrieve all identifiers (ID) of T nodes whose node names are “Shakespeare”, and obtains T 11 , T 31 , T 41 , T 51 , and T 81  ( FIG. 21 ) as a result of this processing.  
      (2) The search control unit  264  refers to the AR table ( FIG. 20 ) stored in the ARDB  240  to retrieve all A node identifiers in which roles are “Author” and T node identifies match the T node identifiers obtained in the processing of (1), and obtains A 1 , A 4  and A 9  as a result of this processing.  
      (3) The search control unit  264  refers to the AR table to retrieve T node identifiers (ID; generally plural) corresponding to A nodes whose association roles R are “Work” among the A node identifiers obtained in the processing of (2), and obtains T 42  and T 92  as a result of this processing.  
      (4) The search control unit  264  refers to the ID table, and sets node names of identifiers corresponding to the T node identifiers obtained by the processing of (3) together with T node identifiers (ID) as search results.  
      That is, the search control unit  264  performs searching based on the search conditions (Work, (NULL), ((Author, Shakespeare))) obtained from the second half of the query statement by the processing of (1) to (4), and obtains search results (T 42 , Hamlet), and (T 92 , the Merchant of Venice).  
      Next, based on the search conditions obtained from the first half of the query statement and the search results corresponding to the second half, the search control unit  264  performs the following.  
      (5) From the search results (T 42 , Hamlet) and (T 92 , the Merchant of Venice) corresponding to the second half of the query statement, the search control unit  264  selects (T 42 , Hamlet) whose node name corresponds to the first search filter (original work, Hamlet), and obtains its node identifier (ID) T 42 .  
      (6) The search control unit  264  refers to the AR table to retrieve all the A node identifiers in which association roles are “Original Work” and T node identifiers match the node identifiers (ID) obtained by the processing of (5), and obtains A 10  and A 19  as a result.  
      (7) The search control unit  264  refers to the AR table to retrieve all identifiers whose roles are “Translation” among the A node identifiers obtained by the processing of (6), and obtains A 10  and A 19  as a result.  
      (8) The search control unit  264  refers to the AR table to retrieve T node identifiers corresponding to the A nodes whose roles are “Publication” among the A node identifiers obtained by the processing of (7), and obtains T 103  as a result (when there are translations from a plurality of publishers, a plurality of T nodes are obtained).  
      (9) The search control unit  264  refers to the ID table, and sets node names of node IDs corresponding to the T node identifiers obtained by the processing of (8) together with node identifiers as search results (T 103 , ∘∘ Publishing Company).  
      That is, the search control unit  264  performs searching based on the search conditions (Publication, (NULL), ((Original Work, Hamlet), (Translation, NULL))) obtained from the first half of the query statement by the processing of (5) to (9), and obtains the search results (T 013 , ∘∘ Publishing Company).  
      (10) The search control unit  264  displays the search results in the input/output device  126  ( FIG. 7 ) or the like to the searcher.  
      [Second Database System  3 ] 
       FIG. 23  shows a configuration of a second DB system  3  in accordance with an embodiment of the present invention.  
      Referring to  FIG. 23 , the second DB system  3  is configured by interconnecting the DB server  12 , a DB server  30  for operating the DB management unit  20  and the DB unit  24  of the DB program  2  on hardware ( FIG. 7 ) similar to the DB server  12 , and a retrieval device  32  for operating the DB search unit  26  of the DB program  2  on hardware similar to the DB server  12  through a network  100 .  
      Thus, the DB program  2  does not need to be always executed on one computer, but it may be distributed to a plurality of computers interconnected through a network to be executed.  
      It should be noted that although some disclosed embodiments of the present invention are implemented in form of software instructions that are contained in a computer-readable medium and executable by a computer, the present invention is not limited to such arrangement. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, the invention is not limited to any specific combination of hardware circuitry and software.