Conjunctive query containment

  • What is a query containment?

    Definition.
    One query is contained in another if, independent of the values of the “stored data” (that is, database), the set of answers to the first query on the database is a subset of the set of answers to the second query on the same database..

A containment mapping from Q1 to Q2 is a mapping from the variables of Q1 to the variables and constants of Q2 that maps each subgoal of Q1 to a subgoal of Q2 and also maps the head of Q1 to the head of Q2. For conjunctive queries, Q1 ⊆ Q2 if and only if there is a containment mapping from Q2 to Q1.
In this lecture we will study when we can say that two conjunctive queries that are syntactically different express the same query, or when the output of one  
The query containment problem for conjunctive queries is also equivalent to the constraint satisfaction problem. in the query) and the conjunctive query is acyclic if and only if its dependency graph is acyclic.

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