Outer Joins and Filters for Instantiating Objects from Relational Databases Through Views

One of the approaches for integrating object-oriented programs with databases is to instantiate objects from relational databases by evaluating view queries. In that approach, it is often necessary to evaluate some joins of the query by left outer joins to prevent information loss caused by the tuples discarded by inner joins. It is also necessary to lter some relations with selection conditions to prevent the retrieval of unwanted nulls. The system should automatically prescribe joins as inner or left outer joins and generate the lters, rather than letting them be speciied manually for every view deenition. We develop such a mechanism in this paper. We rst develop a rigorous system model to facilitate the mapping between an object-oriented model and the relational model. The system model provides a well-deened context for developing a simple mechanism. The mechanism requires only one piece of information from users: null options on an object attribute. The semantics of these options are mapped to non-null constraints on the query result. Then the system prescribes joins and generates lters accordingly. We also address reducing the number of left outer joins and the lters so that the query can be processed more eeciently.