1 research outputs found
A Monotone Preservation Result for Boolean Queries Expressed as a Containment of Conjunctive Queries
When a relational database is queried, the result is normally a relation.
Some queries, however, only require a yes/no answer; such queries are often
called boolean queries. It is customary in database theory to express boolean
queries by testing nonemptiness of query expressions. Another interesting way
for expressing boolean queries are containment statements of the form where and are query expressions. Here, for any input
instance , the boolean query result is if is a
subset of and otherwise.
In the present paper we will focus on nonemptiness and containment statements
about conjunctive queries. The main goal is to investigate the monotone
fragment of the containments of conjunctive queries. In particular, we show a
preservation like result for this monotone fragment. That is, we show that, in
expressive power, the monotone containments of conjunctive queries are exactly
equal to conjunctive queries under nonemptiness