mysql替换表的字段里面内容，如例子：
mysql> select host,user from user where user=‘testuser‘;
+-----------------------+----------+
| host | user |
+-----------------------+----------+
| localhost.localdomain | testuser |
+-----------------------+----------+
update字段host的内容，把"main"改成"slave",用REPLACE
mysql> update user set host=REPLACE(host,‘main‘,‘slave‘) where user=‘testuser‘;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select host,user from user where user=‘testuser‘;
+------------------------+----------+
| host | user |
+------------------------+----------+
| localhost.localdoslave | testuser |
+------------------------+----------+
由查询结果到，数据已经更新成功