数值类型转换问题，当从一个字段从double 变为 float 精度不变，而从float 转变为double 时会发生精度的改变，实验如下：
mysql> create table t2 ( id1 float (5,2) default null, id2 double (5,2) default null, id3 decimal (5,2) default null );
Query OK, 0 rows affected (0.00 sec)
mysql> insert into t2 values (1.23,1.23,1.23);
Query OK, 1 row affected (0.01 sec)
mysql> alter table t2 modify id2 double;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from t2;
+------+------+------+
| id1 | id2 | id3 |
+------+------+------+
| 1.23 | 1.23 | 1.23 |
+------+------+------+
1 row in set (0.01 sec)
mysql> alter table t2 modify id2 float;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from t2;
+------+------+------+
| id1 | id2 | id3 |
+------+------+------+
| 1.23 | 1.23 | 1.23 |
+------+------+------+
1 row in set (0.00 sec)
mysql> desc t2;
+-------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id1 | float(5,2) | YES | | NULL | |
| id2 | float | YES | | NULL | |
| id3 | decimal(5,2) | YES | | NULL | |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql> alter table t2 modify id2 double;
Query OK, 1 row affected (0.01 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from t2;
+------+------------------+------+
| id1 | id2 | id3 |
+------+------------------+------+
| 1.23 | 1.23000001907349 | 1.23 |
+------+------------------+------+
1 row in set (0.01 sec)
mysql>
