So far in this book, we have seen queries that only retrieve data from a single table. However, in the real world, it is very unlikely that you will write queries that only refer to a single table. In practice, the requirement might be to retrieve data from multiple tables. SQL Server 2014 provides several options to create queries that return data from multiple tables. In this section, we will explore these options.
UNION operator is used to combine the result sets of two or more
SELECT statements to generate a single result set. The following is the basic syntax for using the
select_statement UNION [ALL] select_statement [UNION [ALL] select_statement ...