The UNION operator is used to combine the result-set of two or more SELECT statements.
Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.
SQL UNION Syntax
SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;
UNION
SELECT column_name(s) FROM table2;
Note: The UNION operator selects only distinct values by default. To allow duplicate values, use the ALL keyword with UNION.
SQL UNION ALL Syntax
SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;
UNION ALL
SELECT column_name(s) FROM table2;
PS: The column names in the result-set of a UNION are usually equal to the column names in the first SELECT statement in the UNION.