Sql server® 2012 t-sql fundamentals

The UNION Operator

In set theory, the union of two sets (call them A and B) is the set containing all elements of both A and 

B. In other words, if an element belongs to any of the input sets, it belongs to the result set. Figure 6-1 

shows a set diagram (also known as a Venn diagram) with a graphical depiction of the union of two 

sets. The shaded area represents the result of the set operator.

Union: A U B

A

B

FIGuRE 6-1

  A union of two sets.

In T-SQL, the UNION operator unifies the results of two input queries. If a row appears in any of 

the input sets, it will appear in the result of the UNION operator. T-SQL supports both the UNION ALL 

and UNION (implicit DISTINCT) flavors of the UNION operator.