Query Translation . . . into a Relational Query Language

In this paper, we present a query translation algorithm which allows object-oriented queries to be automatically translated into a relational query language. Our goal is to provide an improved query interface for existing relational database systems. The translation algorithm, we propose in this paper may be used to directly access relational databases, but it may also be useful in the context of object-oriented multidatabase systems to translate the common global query language into the query languages of participating relational databases. Necessary steps in providing object-oriented access to relational databases are schema enrichment and transformation as well as query translation. The main focus of this paper is the query translation which has to be performed fully automatically since it has to be done each time, a query is processed by the system, whereas schema enrichment and transformation may be done only once in the beginning. Our query translation algorithm ensures a full automatic translation of object-oriented queries into equivalent SQL queries for the original relational schema in all cases where a direct translation is possible. In all other cases, it generates SQL queries providing a superset of the desired data and a sequence of ‘formatting’ functions that transform the data into the desired result. Problems may occur if additional user defined functions are used.