On keys, foreign keys and nullable attributes in relational mapping systems

We consider the following scenario for a mapping system: given a source schema, a target schema, and a set of value correspondences between these two schemas, generate an executable transformation (i.e., a set of queries) to compute target instances from source instances. We base this computation on two main components: (i) a schema mapping generation algorithm, to compute a declarative schema mapping from the correspondences, and (ii) a query generation algorithm, to compute a transformation from the schema mapping. In this paper, we introduce novel schema mapping and query generation algorithms for mappings between relational schemas with keys, foreign keys and nullable attributes. We extend current relational mapping algorithms (e.g., those proposed in the Clio framework), which are able to deal only in a more limited way with such integrity constraints. As a further contribution, we propose referenced-attribute correspondences, which permit to specify more precise mappings than traditional attribute correspondences, while retaining a simple and intuitive semantics.