Semantics of query languages for network databases

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Abstract
Semantics determines the meaning of language constructs; hence it says much more than syntax does about implementing the language. The main purpose of this paper is a formal presentation of the meaning of basic language constructs employed in many database languages (sublanguages). Therefore, stylized query languages SSL (Sample Selection Language) and J (Joins) are introduced, wherein most of the typical entries present in other query languages are collected. The semantics of SSL and J are defined by means of the denotational method and explained informally. In SSL and J, four types of expressions are introduced: a selector (denotes a set of addresses), a term (denotes a set of values), a formula (denotes a truth value), and a join (denotes a set of n-tuples of addresses or values). In many cases alternative semantics are given and discussed. In order to obtain more general properties of the proposed languages, a new database access model is introduced, intended to be a tool for the description of the logical access paths to data. In particular, the access paths of the network and relational models can be described. SSL and J expressions may be addressed to both data structures. In the case of the relational model, expressions of J are similar to SQL or QUEL statements. Thus J may be considered a generalization of relational query languages for the network model. Finally, a programming language, based on SSL and J, is outlined, and the issues of SSL and J implementation are considered.

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