UNION

UNIONで、複数のSQL実行結果の「和」を返します。

構文:

<SQL文1> union <SQL文2>;

サンプル:

select name from kittens where id between 1 and 2 
union 
select name from kittens where id between 2 and 3;

データ:

mysql> select * from kittens;
+----+-------+-----------+
| id | name  | friend_id |
+----+-------+-----------+
|  1 | mii   |         3 |
|  2 | tora  |         3 |
|  3 | shiro |         1 |
|  4 | kuro  |      NULL |
+----+-------+-----------+
4 rows in set (0.01 sec)

サンプルの実行結果:

mysql> select name from kittens where id between 1 and 2 union select name from kittens where id between 2 and 3;
+-------+
| name  |
+-------+
| mii   |
| tora  |
| shiro |
+-------+
3 rows in set (0.00 sec)

実行結果の比較を行うため、UNIONするSQLは同じカラムを返すSQL同士でなければなりません。カラムの数が違ったりする場合、エラーです。

mysql> select name from kittens where id = 1 union select * from kittens where id = 2;
ERROR 1222 (21000): The used SELECT statements have a different number of columns

参考:"TECHSCORE-SQL-クエリの結合"