也议MySQL中隐式转换 - 数据库编程

-------------+ 1 row in set (0.00 sec)

MySQL [test]> select * from emp where ename=23456;   ## 当varchar类型不对时，仍然是可以查出结果
+-------+-------+-----------+------+---------------------+------+------+--------+
| EMPNO | ENAME | JOB       | MGR  | HIREDATE            | SAL  | COMM | DEPTNO |
+-------+-------+-----------+------+---------------------+------+------+--------+
|  7936 | 23456 | BOSCO-DBA | 7788 | 2014-09-13 16:13:56 | 2450 |  800 |     10 |
+-------+-------+-----------+------+---------------------+------+------+--------+
1 row in set, 14 warnings (0.00 sec)

MySQL [test]> explain select * from emp where ename=23456;   ## 当varchar类型不匹配时，索引无效了，选择了全表扫描
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | emp   | ALL  | idx_ename     | NULL | NULL    | NULL |   15 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

可见，如果是字符类型，当出现类型不一致时，是会影响索引的使用的，会产生隐式转换的。

blog地址：http://blog.csdn.net/hw_libo/article/details/39252427

-- Bosco QQ：375612082

也议MySQL中隐式转换(三)