These are the binary operations; i.e., each is applied to two relations or sets. These two relations can be union compatible excluding in case of Cartesian product. Two relations R (A_{1}, A_{2},....., A_{n}) and S (B_{1}, B_{2},......,B_{n}) are said to be union compatible if they have the similar degree n and domains of the corresponding attributes are also the similar
i.e. Domain (Ai) = Domain (Bi) for 1<=i<=n.