The goal of the work reported here is to enable a database user to confidently get correct answers to many queries that previously were beyond reach. They grew out of a study of relational algebra operator division.
Division is hard to understand, and is both underutilized and insufficient.
These new operators are important to DBMS users and vendors. Users now can perform HAS with existing DBMS's by using a dozen or so operators. To accomplish the more complex tasks of Vary-HAS, an implementation must treat attribute values as command clauses. Commercially available DBMS's, if they are to serve more demanding applications and more sophisticated users, will need to do so too. There are special cases with 2 or 3 relations.