  Cooperative database systems support a database user by searching for answers
that are closely related to his query and hence are informative answers. Common
operators to relax the user query are Dropping Condition, Anti-Instantiation
and Goal Replacement. In this article, we provide an algebraic version of these
operators. Moreover we propose some heuristics to assign a degree of similarity
to each tuple of an answer table; this degree can help the user to determine
whether this answer is relevant for him or not.
