The purpose of the relational model is to provide a 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.
IBM's original implementation of Codd's ideas was System R. There have been several commercial and open source products based on Codd's ideas, including IBM's DB2,Oracle Database, Microsoft SQL Server, PostgreSQL, MySQL, and many others. Most of these use the SQL data definition and query language. 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, including DB2, deviate from the relational model in many details; Codd fiercely argued against deviations that compromise the original principles
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