This command combines results from two or more select queries. In order to use the UNION command, you must have the same number of columns in each select statement and the corresponding columns must have the same data type.
Note: Union removes duplicate rows (unless UNION ALL is used)
Combining entire customer e-mail list from two separate stores (note: each customer will be listed only once, even if they signed up for e-mails in both stores):
SELECT customer_id, email FROM store1
SELECT customer_id, email FROM store2
Combining sales figures from two different databases (note: UNION ALL is used in case an employee sold the same amount in both quarters:
SELECT employee, amount FROM salesQ1
SELECT employee, amount FROM salesQ2;