# Semantics-Preserving RDB2 RDF Data Transformation Using Hierarchical Direct Mapping
## 2 Related Work  
RDB2 RDF is a mapping method that transforms relational data into semantic data represented by the RDF. The RDF data model [17] is a language that describes semantic information on the Semantic Web. The basic unit of RDF data is based on a graph structure (i.e., the triple: subject, property, and object) [18; 19; 20]. The RDF is a flexible and interoperable model used to publish information to the web relative to the relational data model. However, because ~70% of websites are backed up as RDBs [1], existing relational data must be used with the RDB2 RDF methodology for the improvement of the Semantic Web. RDB2 RDF mapping approaches include mapping creation, mapping representation and accessibility, mapping implementation, query implementation, application domain, and data integration [21]. Mapping creation has been widely studied to improve the generation of mappings between relational data and semantic data, and it can be performed either automatically or manually [22] (see Table 1 for a list of previous mapping works). Domain semantics-driven mapping is a manual mapping method [23; 24]. The W3C RDB2 RDF Working Group has recommended the R2 RML mapping language [25; 26] for customizing mappings. Mapping tools, such as D2 RQ [27; 28], Virtuoso [29; 30], Ultrawrap [31; 32], etc. have also been provided to support manual mapping. On the other hand, direct mapping is an automatic mapping method that was published by the W3C RDB2 RDF Working Group (see Table 1 for a list of previous mapping works). Domain semantics-driven mapping is a manual mapping method [23; 24]. The W3C RDB2 RDF Working Group has recommended the R2 RML mapping language [25; 26] for customizing mappings. Mapping tools, such as D2 RQ [27; 28], Virtuoso [29; 30], Ultrawrap [31; 32], etc. have also been provided to support manual mapping. On the other hand, direct mapping is an automatic mapping method that was published by the W3C RDB2 RDF Working Group (see Table 1 for a list of previous mapping works). Domain semantics-driven mapping is a manual mapping method [23; 24]. The W3C RDB2 RDF Working Group has recommended the R2 RML mapping language [25; 26] for customizing mappings. Mapping tools, such as D2 RQ [27; 28], Virtuoso [29; 30], Ultrawrap [31; 32], etc. have also been provided to support manual mapping. On the other hand, direct mapping is an automatic mapping method that was published by the W3C RDB2 RDF Working Group (see Table 1 for a list of previous mapping works). Domain semantics-driven mapping is a manual mapping method [23; 24]. Figure 3: Overview of our direct-mapping method. Group in 2012 [5]. It uses RDB instances and schemas as inputs and automatically generates RDF semantic data. In the field of RDF data creation, some methods used to transform various types of data (e.g., heterogeneous data [33], object-oriented data [34], and the Web of Data [35]) to RDF data have been devised. The current paper, however, mainly focuses on direct mapping to manage large-scale data on the web. Further research has been conducted to obtain semantic data from relational data without information loss [6; 8; 9; 10; 11; 12; 13; 14; 15; 6]. RDF schema (RDFS) and Web Ontology Language (OWL) are used to obtain a more accurate mapping for RDB2 RDF transformation. The concept of RDF data can be modeled by RDFS or OWL in a manner similar to that used for defining the relational schema using SQL data definition language (DDL). Moreover, because OWL contains a more expressive semantic vocabulary, the mapping methods can better express the semantics of relational integrity constraints. [PERSON] et al. [7] proposed an augmented direct-mapping method that generates semantic data from the integrity constraints of the SQL DDL schema. Because integrity constraints define the semantics of the RDBs, the quality of augmented direct mapping depends on the transformation of the integrity constraints of the RDBs. DB2 OWL [10] and RDBToOnto [11] also provided augmented direct mapping tools.