Visualization of path expressions in a visual object-oriented database query language

Abstract

Path expressions have been accepted for concisely manipulating the nested structures in complex object-oriented query expressions. However, previous visual query languages hardly represent such query expressions in a concise and intuitive way partly due to improper visual representation of path expressions and partly due to lack of well-defined system and semantics of languages. In this paper, we present visual modeling of path expressions in a visual object-oriented database query language called Visual Object-Oriented Query Language (VOQL) which has excellent expressive pourer for sets, simple and intuitive syntax, and well-defined semantics. This is enabled by explicitly specifying the semantics of multi-valued path expressions based on the visual notation capable of representing set relationships in addition to functional relationships. The basic visual constructs called blobs and nested blobs denote sets of objects that path expressions represent while the constructs called binding edges and flattening edges visually simulate the notions of variable binding and dot functions in path expressions respectively. Based on the constructs, the grammar of VOQL defines the syntactic components while the semantics of query expressions are provided by syntax-directed translation to the counterparts in the extended relational calculus. Also, the visual constructs allow modeling of restricted universal quantification with a visual scoping box and effectively represent nested quantification and recursive queries without semantic ambiguities.