  Since Chen's Entity-Relationship (ER) model, conceptual modeling has been
playing a fundamental role in relational data design. In this paper we consider
an extended ER (EER) model enriched with cardinality constraints, disjointness
assertions, and is-a relations among both entities and relationships. In this
setting, we consider the case of incomplete data, which is likely to occur, for
instance, when data from different sources are integrated. In such a context,
we address the problem of providing correct answers to conjunctive queries by
reasoning on the schema. Based on previous results about decidability of the
problem, we provide a query answering algorithm that performs rewriting of the
initial query into a recursive Datalog query encoding the information about the
schema. We finally show extensions to more general settings. This paper will
appear in the special issue of Theory and Practice of Logic Programming (TPLP)
titled Logic Programming in Databases: From Datalog to Semantic-Web Rules.
