DOI: 10.21276/sjet

Pages: 254-257

The paper considers inner and outer join of relation operations, as well as set-theoretical operations on relations. Unlike join operations in Codd’s relational algebra the join operations under consideration are based on Kleene and Priest three-valued logic used in SQL, that allows to give the semantics of join operations more precisely. In relational algebra, any relation that does not contain tuples is represented as an empty set. It is impossible to define outer joins for such relations provided we do not know what columns contain an empty relation. In the paper relations with schemas are introduced. That allows the operations to be defined even if one or both operands are empty relations unlike classical relational algebra.