update 子查询 exists

要根据一个表的数据订正另一个表的部分,

例如，要订正xy1表的字段a为xy2表中的字段a的值，即，订正表xy1中id为1，2，3的记录的a字段的值，id为4，5的记录保持不变。可用uodate exists语句：

mysql> select * from xy1;
+----+------+------+
| id | a    | b    |
+----+------+------+
|  1 | t1   |    3 |
|  2 | t2   |    4 |
|  3 | t3   |    5 |
|  4 | t4   |    8 |
|  5 | t5   |    7 |
+----+------+------+
5 rows in set (0.07 sec)

mysql> select * from xy2;
+----+------+
| id | a    |
+----+------+
|  1 | y10  |
|  2 | y20  |
|  3 | y30  |
+----+------+
3 rows in set (0.00 sec)

mysql>  update xy1 set a=(select a from xy2 where xy1.id=xy2.id) where exists(select 1 from xy2 where xy1.id=xy2.id);
Query OK, 3 rows affected (0.01 sec)
Rows matched: 3  Changed: 3  Warnings: 0

mysql> select * from xy1;
+----+------+------+
| id | a    | b    |
+----+------+------+
|  1 | y10  |    3 |
|  2 | y20  |    4 |
|  3 | y30  |    5 |
|  4 | t4   |    8 |
|  5 | t5   |    7 |
+----+------+------+
5 rows in set (0.03 sec)

mysql> select * from xy2;
+----+------+
| id | a    |
+----+------+
|  1 | y10  |
|  2 | y20  |
|  3 | y30  |
+----+------+
3 rows in set (0.03 sec)