MySQL の UNION

SELECT COLUMN_LIST FROM FIRST_TABLE
UNION
SELECT COLUMN_LIST FROM SECOND_TABLE;

SELECT rating FROM film UNION SELECT rating FROM film;

+--------+
| rating |
+--------+
| PG     |
| G      |
| NC-17  |
| PG-13  |
| R      |
+--------+
5 rows in set (0.00 sec)

SELECT rating FROM film UNION ALL SELECT rating FROM film;

+--------+
| rating |
+--------+
| PG     |
| G      |
| NC-17  |
...
+--------+
2000 rows in set (0.00 sec)

SELECT 'staff' AS source, first_name, last_name FROM staff
UNION ALL
SELECT 'actor', first_name, last_name FROM actor WHERE actor_id <= 3;

+--------+------------+-----------+
| source | first_name | last_name |
+--------+------------+-----------+
| staff  | Mike       | Hillyer   |
| staff  | Jon        | Stephens  |
| actor  | PENELOPE   | GUINESS   |
| actor  | NICK       | WAHLBERG  |
| actor  | ED         | CHASE     |
+--------+------------+-----------+
5 rows in set (0.00 sec)

SELECT first_name FROM actor UNION SELECT first_name, last_name FROM actor;

ERROR 1222 (21000): The used SELECT statements have a different number of columns

SELECT first_name AS name FROM staff
UNION
SELECT last_name FROM staff
ORDER BY name;

+----------+
| name     |
+----------+
| Hillyer  |
| Jon      |
| Mike     |
| Stephens |
+----------+
4 rows in set (0.00 sec)