MYSQL 动态变量赋值不对的情况

那天我碰到了个怪问题。今天解决了问题。
不过具体原因还有待查找。
在WINDOWS下正常。

DROP TABLE IF EXISTS `t`;

CREATE TABLE `t` (
  `id` int(11) NOT NULL auto_increment,
  `g_id` int(11) NOT NULL,
  `t_str` varchar(255) NOT NULL,
  PRIMARY KEY  (`id`),
  key (`g_id`)
) ENGINE=InnoDB AUTO_INCREMENT=26 DEFAULT CHARSET=utf8;

/*Data for the table `t` */

insert  into `t`(`id`,`g_id`,`t_str`) values
(1,2,'wo'),
(2,2,'ni'),
(3,2,'ta'),
(4,3,'wo '),
(5,4,'ni'),
(6,3,'ni'),
(7,4,'ta'),
(8,3,'wang'),
(9,4,'li'),
(10,3,'hai'),
(11,4,'ri'),
(12,2,'ren'),
(13,5,'ta'),
(14,6,'ri'),
(15,6,'ren'),
(16,6,'fuck'),
(17,6,'shit'),
(18,5,'ls'),
(19,5,'chmod'),
(20,5,'chgrp'),
(21,5,'chown'),
(22,3,'rm'),
(23,3,'desc'),
(24,4,'pwd'),
(25,5,'cd');

错误输出

mysql>  select g_id from t group by g_id order by g_id desc limit 0,1;
+------+
| g_id |
+------+
|    6 |
+------+
1 row in set (0.00 sec)

mysql>  select g_id from t group by g_id order by g_id desc limit 0,1 into @tmp_id;
Query OK, 1 row affected (0.00 sec)

mysql> select @tmp_id;
+---------+
| @tmp_id |
+---------+
|       5 |
+---------+
1 row in set (0.00 sec)

mysql> select version();
+-----------------+
| version()       |
+-----------------+
| 5.1.21-beta-log |
+-----------------+
1 row in set (0.00 sec)

mysql>

正确输出：

mysql> select @tmp_id := g_id from t group by g_id order by g_id desc limit 0,1 into @tmp_id;
Query OK, 1 row affected (0.00 sec)

mysql> select @tmp_id;
+---------+
| @tmp_id |
+---------+
| 6       |
+---------+
1 row in set (0.00 sec)