Relationcondition

Relationcondition is a term used in data management and logic to denote a constraint or predicate that ties together two or more relations by asserting a relation between their attributes. It is a general concept for expressing how data in distinct relations should correspond to each other, and it underpins join operations, query predicates, and certain data-integrity rules.

It can take several forms. A simple equality predicate between shared attributes constitutes an equi-join condition, such as R.x = S.y. More generally, a relationcondition can be a theta predicate using operators such as <, >, <=, >=, !=, or IN. In relational databases, a relationcondition often appears as the ON clause of a join or as a filter in a WHERE clause; in relational algebra, it is the predicate that guides the join.

As semantics, a relationcondition is evaluated against a particular database state. A pair of tuples from the involved relations satisfies the condition if the relation between the chosen attributes holds. For join-type uses, the set of all pairings that satisfy the condition forms the result of the join. In integrity constraints, a relationcondition expresses dependencies across relations, such as foreign-key-like references.

Relationcondition is closely related to terminology in databases and logic, including join predicates, theta joins, equijoins, referential integrity, and foreign keys. It is a foundational concept for querying and validating relational data, and it generalizes to higher-arity predicates that relate more than two relations or attributes.