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The purpose of the relational model is to provide a [[Declarative programming |declarative]] method for specifying data and queries: users directly state what information the database contains and what information they want from it, and let the database management system software take care of describing data structures for storing the data and retrieval procedures for answering queries.
Most relational databases use the [[SQL]] data definition and query language; these systems implement what can be regarded as an engineering approximation to the relational model. A ''table'' in an SQL database schema corresponds to a predicate variable; the contents of a table to a relation; key constraints, other constraints, and SQL queries correspond to predicates. However, SQL databases [[#SQL and the relational model|deviate from the relational model in many details]], and Codd fiercely argued against deviations that compromise the original principles.<ref>{{Citation | first = E. F | last = Codd | title = The Relational Model for Database Management | publisher = Addison-Wesley | year = 1990 | ISBN = 0-201-14192-2 | pages=371-388}}.</ref>
== Overview ==
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