Visual Query Systems

The widespread dif fusion of the computer-based knowledge has , among others , the ef fect of providing everybody with the possibility of directly accessing information resources through database technology . On the one side , this is extremely useful and exciting . On the other side , the ever growing amount of information at disposal generates cognitive overload and even anxiety , especially in novice or occasional users . Interfaces for interacting with databases are usually too hostile for novices and / or inadequate for experts . The problem of easily extracting information from databases has been discussed extensively among the database community . Several query modalities , representing alternatives to traditional query languages such as SQL , have been proposed recently . In particular , Visual Query Systems (VQSs) are based on the use of visual representations to depict the domain of interest , and direct manipulation interaction mechanisms in order to formulate requests to the database by means of visual expressions . VQSs take advantage of the well-known high bandwidth of the human-vision channel , allowing both recognition and handling of large quantities of information as well as the possibility of using visual feedback to improve the human – computer dialogue . VQSs provide both a language to express the queries in a visual format and variety of functionalities to facilitate user – system interaction . As such , they are oriented towards a wide spectrum of users , especially novices who have limited computer expertise and generally ignore the inner structure of the accessed database . In recent years , several VQSs have been proposed in the literature . They adopt a range of dif ferent visual representations and interaction strategies . Visual representations are ef fective for expressing dif ferent kinds of knowledge . One of the main advantages relies on their capacity to allow perceptual interferences , replacing arduous cognitive comparisons and computations . The most used visual representations are forms , diagrams and icons . Each of these presents peculiar features . For instance , forms easily represent common properties of collections of objects , diagrams ef fectively display relationships among objects , and icons supply immediately graspable representations of object classes . This growing interest was reflected by the good response to our call for papers for a special issue on VQSs . We received several good papers and we could accomodate four of them in the first part of this special issue , published in the March 1995 number of this journal . Three other papers that received favorable evaluation from the reviewers are now presented . The first paper , ‘A hybrid query language for an extended Entity-Relationship model’ , by Andries and Engels , presents the hybrid query language HQL / EER , where queries are expressed by means of a mixture of graphical and textual elements . The advantage of such a hybrid language is to allow users the graphical formulation of those parts of a query that are more easily formulated graphically than textually , and vice versa , so providing users with ‘the best of both worlds’ . Despite most VQSs that