Extensions of Conjunctive Queries
Extensions of conjunctive queries capturing more expressive power include unions of conjunctive queries, which are equivalent to positive (i.e., negation-free) relational algebra, conjunctive queries extended by union and negation, which by Codd's theorem correspond to relational algebra and first-order logic, conjunctive queries with built-in predicates and conjunctive queries with aggregate functions. The formal study of all of these extensions is justified by their application in relational databases and is in the realm of database theory.
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