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 a 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 deviate from the relational model in many details, and Codd fiercely argued against deviations that compromise the original principles..
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 a 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 deviate from the relational model in many details and Codd fiercely argued against deviations that compromise the original principles