Visual query languages, representation techniques and data models

An easy, efficient and effective way to retrieve stored data is obviously one of the key issues of any information system. In the last few years considerable effort has been devoted to the definition of more intuitive; visual-based querying paradigms; attempting to offer a good trade-off between expressiveness and intuitiveness. In this chapter we analyze the main characteristics of visual languages specifically designed for querying information systems; concentrating on conventional relational databases but also considering information systems with a less rigid structure; such as Web resources storing XML documents. We consider two fundamental aspects of visual query languages: the adopted visual representation technique and the underlying data model; possibly specialized to specific application contexts.