The union operation is a binary operation that requires two relations as its operands. It produces a third relation that contains tuples from both the operand relations. The union operation is denoted by ∪ . Thus, to denote the union or two relations X and Y, we will write as X ∪ Y.
Following two condition must be satisfied by two operands A and B :
(i) The relations A and B must be of the same degree. That is, they must have the same number of attributes.
(ii) The domains of the ith attributes of A and the ith attributes of B must be the same.
