Queries in object-oriented database systems

Abstract

A query language is an important component of any database system. In object-oriented data models, although messages serve to manipulate the database, a query language is still required to effectively deal with more complex situations and to facilitate associative access. In this paper, a query language for object-oriented data models is described. It handles both the structure as well as the behavior of objects. Not only the manipulation of existing objects, but also the creation of new objects and the introduction of new relationships are supported by the language. Equivalents to the five basic operations of the relational model as well as other additional operations such as one level project and nest are defined. Both the operands as well as the results of these operations are characterized as having a pair of sets -a set of objects and a set of message expressions (sequences of messages) applicable to them. The closure property is shown to be preserved in a natural way by the results of operations possessing the same characteristics as the operands in a query. It is shown that every class possesses the properties of an operand by defining a set of objects and deriving a set of message expressions for it. It is also shown how the super♪bclass relationships of the result of a query with its operands can be established and how the result can be placed persistently in the lattice as a class.