Visual query systems (VQSs) are query systems for databases that use visual representations to depict the domain of interest and express related requests. VQSs can be seen as an evolution of query languages adopted into database management systems; they are designed to improve the effectiveness of the human-computer communication. Thus, their most important features are those that determine the nature of the human-computer dialogue. In order to survey and compare existing VQSs used for querying traditional databases, we first introduce a classification based on such features, namely the adopted visual representations and the interaction strategies. We then identify several user types and match the VQS classes against them, in order to understand which kind of system may be suitable for each kind of user. We also report usability experiments which support our claims. Finally, some of the most important open problems in the VQS area are described. 1.

Abstract. The extended NF 2 (eNF 2) data model permits structured data values like (sub-)relations, lists and tuples to be included as attributes in relations. While it is a very powerful data model, it is not easy to handle and even harder to implement. In