UNION 

The keyword UNION allows constructing the union of multiple query specifications. Duplicate rows are implicitly eliminated from the result set unless UNION ALL is specified. In addition, you can order the result of a union using the ORDER BY clause.

Syntax

<query expression> ::= <query term>                                   | <query expression> UNION ( ALL )? <query term>. <query term>::= <query specification>                        | '(' <query expression> ')'.

 

Examples

SELECT employee_name FROM employees UNION SELECT name FROM more_employees

UNION. This SELECT statement determines the union of all the names of all employees from the tables employees and more_employees. The result set will not contain duplicate names.

SELECT empid AS employee_id, sal AS salary FROM employees UNION SELECT empid AS employee_id, sal AS salary FROM more_employees ORDER BY employee_id

UNION with an ORDER BY Clause. The result of the query is ordered by employee ID in ascending order. Note that the column name from the result table (employee_id) is used in the ORDER BY clause.

More Information

Query Specification