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. Today, many database systems are different from the original model. Codd fiercely argued against deviations that compromise the original principles.