关于update关联表的写法存在很多误区,以前我自己也经常犯错....
一般的写法有如下几种:
update test1 set name =(select name from test2 where test1.id=test2.id);
update test1 a,test2 b set a.name=b.name where a.id=b.id;
update test2 set test2.name=(select name from test1 where test1.id=test2.id) where exists (select * from test1 where test1.id=test2.id);

一般来讲这三种写法都没问题,只要在test1,和test2中name和id字段都是唯一的...
下面我们分别讨论更改表和关联表的情况:
首先,讨论被关联表有重复的情况:

mysql> select * from test1;
+------+------+
| id   | name |
+------+------+
|    1 | k1   |
|    2 | k2   |
|    3 | k3   |
|    4 | kkk  |
|    4 | k4   |
|    4 | k4   |
+------+------+
6 rows in set (0.00 sec)

mysql> select * from test2;
+------+------+
| id   | name |
+------+------+
|    1 | 2222 |
|    2 | 2222 |
|    3 | 2222 |
|    4 | 2222 |
+------+------+
4 rows in set (0.00 sec)

mysql> update test2 set name = (select test1.name from test1 where test1.id=test2.id)   
    -> ;
ERROR 1242 (21000): Subquery returns more than 1 row


mysql> update test2 a,test1 b set a.name=b.name where a.id=b.id;
Query OK, 4 rows affected (0.01 sec)
Rows matched: 4  Changed: 4  Warnings: 0

mysql> select * from test2;
+------+------+
| id   | name |
+------+------+
|    1 | k1   |
|    2 | k2   |
|    3 | k3   |
|    4 | kkk  |
+------+------+
4 rows in set (0.00 sec)

test1表中id=4的记录有多个...且值也不一致.这个时候将test1做为驱动表,是不可取的..不管用什么语法,要么会报错,要么就会只去驱动表的重复的值的第一个值...




接着讨论被更改表有多余值的情况
多余值是相对于更改条件而言的.比如只更改ID1-4的..而存在id=5的类似的情况;
mysql> select * from test2;
+------+---------+
| id   | name    |
+------+---------+
|    1 | k1      |
|    2 | k2      |
|    3 | k3      |
|    4 | kkk     |
|    5 | gaopeng |
+------+---------+
5 rows in set (0.00 sec)

mysql> select * from test1;
+------+------+
| id   | name |
+------+------+
|    1 | k1   |
|    2 | k2   |
|    3 | k3   |
+------+------+
3 rows in set (0.00 sec)

mysql> update test2 set test2.name=(select name from test1 where test1.id=test2.id);
Query OK, 2 rows affected (0.01 sec)
Rows matched: 5  Changed: 2  Warnings: 0

mysql> select * from test2;
+------+------+
| id   | name |
+------+------+
|    1 | k1   |
|    2 | k2   |
|    3 | k3   |
|    4 | NULL |
|    5 | NULL |
+------+------+
5 rows in set (0.00 sec)

可以看到,如果不加条件的update,会导致多余的数据被更改为null.
mysql> select * from test2;
+------+------+
| id   | name |
+------+------+
|    1 | rrr  |
|    2 | rrr  |
|    3 | rrr  |
|    4 | rrr  |
|    5 | rrr  |
+------+------+
5 rows in set (0.00 sec)

mysql> update test2 set test2.name=(select name from test1 where test1.id=test2.id) where exists (select * from test1 where test1.id=test2.id); 
Query OK, 3 rows affected (0.01 sec)
Rows matched: 3  Changed: 3  Warnings: 0

mysql> select * from test2;
+------+------+
| id   | name |
+------+------+
|    1 | k1   |
|    2 | k2   |
|    3 | k3   |
|    4 | rrr  |
|    5 | rrr  |
+------+------+
5 rows in set (0.00 sec)

所以要注意关联更改时要注意的条件:

1.驱动表不能有重复值.关联表作为参考值,不能有重复,不然取值时不知道取哪一个值
2.被更改表如有多余值时,一定要加条件,不然,没有被关联到的数据会被更改为null;