High Quality Content by WIKIPEDIA articles! Tuple calculus is a calculus that was introduced by Edgar F. Codd as part of the relational model, in order to provide a declarative database-query language for this data model. It formed the inspiration for the database-query languages QUEL and SQL, of which the latter, although far less faithful to the original relational model and calculus, is now the de-facto-standard database-query language; viz., a dialect of SQL is used by nearly every relational-database-management system. Lacroix and Pirotte proposed domain calculus, which is closer to first-order logic and which showed that both of these calculi (as well as relational algebra) are equivalent in expressive power. Subsequently, query languages for the relational model were called relationally complete if they could express at least all of these queries.