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 formal definition of containment is as follows: denote with Q(D) the result of computing query Q over database D. A query Q1 is said to be contained in a query Q2, denoted by Q1 ⊑ Q2, if for all databases D, the set of tuples Q1(D) is a subset of the set of tuples Q2(D), that is, Q1(D) ⊆ Q2(D). This definition of containment, as well as the related definition of query equivalence, can be used to specify query containment and equivalence on databases conforming to both relational and nonrelational data models, including XML and object-oriented databases.
Testing for query containment on finite databases is, in general, co-recursively enumerable: The procedure is going through all possible databases and simultaneously...
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