mysql> select * from test;
+------+------+
| id   | col2 |
+------+------+
|    1 |    2 |
|    2 |    1 |
|    1 |    3 |
|    3 |   14 |
+------+------+
4 rows in set (0.00 sec)

mysql> select max(col2) into @col2 from test;update test set col2=@col2+10 where id=3;
Query OK, 1 row affected (0.00 sec)

Query OK, 1 row affected (0.03 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from test;
+------+------+
| id   | col2 |
+------+------+
|    1 |    2 |
|    2 |    1 |
|    1 |    3 |
|    3 |   24 |
+------+------+
4 rows in set (0.00 sec)

------解决方案--------------------
UPDATE fanwe_share,(select max(px)+1 AS MA from fanwe_share) B
SET A.px =B.MA WHERE share_id = 2

------解决方案--------------------
探讨


还有问一下，
我想先用SELECT得到的值赋给变量，然后UPDATE直接调用变量，，但下面这样定不行耶，好像每条语句用“;”结束后下一条语句就调用不了这个语句的变量了。是不是呀，，其实用触发器来解决最好了，但我不会，。。。
select @id=max(px)  as a  from  fanwe_share;
UPDATE fanwe_share SET px =@id+1 WHERE ……


------解决方案--------------------
select @id:=max(px) from fanwe_share;
UPDATE fanwe_share SET px=@id+1 WHERE share_id = 2;

这样不行么，你试过了？ @id:=