-----+-------+---------+------+---------------------+------+------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |
+-------+-------+---------+------+---------------------+------+------+--------+
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000 | NULL | 20 |
+-------+-------+---------+------+---------------------+------+------+--------+
1 row in set (0.00 sec)
MySQL [test]> explain select * from emp where empno='7788';
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | emp | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec) 可见，针对数据类型字段，即使类型不一致，并不影响是否使用索引，执行计划是一样的，不会产生隐式转换。但仍然建议在生产库中尽量避免出现这样的SQL。
注意：
数值类型有一种隐式转换，如果以数字开关的，后面的字符将被截断，只取前面的数字值，如果不以数字开关的将被置为0。如下：
MySQL [test]> select * from emp where empno='7788ab12'; ## 这个就相当于empno=7788，后面的ab12将被截断，并且不影响索引的使用
+-------+-------+---------+------+---------------------+------+------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |
+-------+-------+---------+------+---------------------+------+------+--------+
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000 | NULL | 20 |
+-------+-------+---------+------+---------------------+------+------+--------+
1 row in set, 1 warning (0.00 sec)
MySQL [test]> show warnings;
+---------+------+----------------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: '7788ab12' |
+---------+------+----------------------------------------------+
1 row in set (0.00 sec)
MySQL [test]> select * from emp where empno='ab7788'; ## 这个就相当于empno=0
Empty set (0.01 sec)
3. 字符类型（varchar）
同样，针对测试表emp中的ename字段（varchar类型），上面有一辅助索引idx_ename，并且ename中有一个值是全数字的，若有这样的查询：
select * from emp where ename=23456;
上面的SQL会不会出现隐式转换呢？
下面实验证明：
MySQL [test]> select * from emp where ename='23456';
+-------+-------+-----------+------+---------------------+------+------+--------+
| 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 (0.00 sec)
MySQL [test]> explain select * from emp where ename='23456'; ## 正常来说，可以使用到索引idx_ename
+----+-------------+-------+------+---------------+-----------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-----------+---------+-------+------+-----------------------+
| 1 | SIMPLE | emp | ref | idx_ename | idx_ename | 47 | const | 1 | Using index condition |
+----+-------------+-------+------+---------------+-----------+---------+-------+------+----------