UNION (Relational Operation)

The binary operation UNION, denoted by $\cup$, creates a relation that includes all tuples that includes all tuples from either operand.

The result of $R\cup S$ is a relation that includes all tuples that are either in $R$, in $S$, or in both. Duplicate tuples are eliminated.

Examples:

• Retrieve the SSNs of all employees who work in dept 5. or directly supervise an employee who works in dept 5. $$\begin{array}{rl}DEP{5}_{EMPS}& \leftarrow {\sigma }_{DNO=5}(EMPLOYEE)\\ RESUL{T}_1& \leftarrow {\pi }_{SSN}(DEP{5}_{EMPS})\\ RESUL{T}_2(SSN)& \leftarrow {\pi }_{SUPERSSN}(DEP{5}_{EMPS})\\ RESULT& \leftarrow RESUL{T}_1\cup RESUL{T}_2\end{array}$$