blog地址：http://blog.csdn.net/hw_libo/article/details/39252427
RHEL 6.4 x86_64 + MySQL 5.6.19
测试表：
MySQL [test]> show create table emp\G
*************************** 1. row ***************************
Table: emp
Create Table: CREATE TABLE `emp` (
`EMPNO` int(11) NOT NULL,
`ENAME` varchar(15) NOT NULL,
`JOB` varchar(15) NOT NULL,
`MGR` int(11) DEFAULT '0',
`HIREDATE` timestamp NULL DEFAULT NULL,
`SAL` int(20) DEFAULT '0',
`COMM` int(11) DEFAULT '0',
`DEPTNO` int(11) NOT NULL,
PRIMARY KEY (`EMPNO`),
KEY `idx_deptno` (`DEPTNO`),
KEY `idx_sal` (`SAL`),
KEY `idx_comm` (`COMM`),
KEY `idx_ename` (`ENAME`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)MySQL [test]> select * from emp; +-------+--------+-----------+------+---------------------+------+------+--------+ | EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | +-------+--------+-----------+------+---------------------+------+------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600 | 300 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250 | 500 | 30 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975 | 0 | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250 | 1400 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850 | 0 | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450 | 0 | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000 | NULL | 20 | | 7839 | KING | PRESIDENT | 0 | 1981-11-17 00:00:00 | 5000 | 0 | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500 | 0 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100 | 0 | 20 | | 7900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950 | 0 | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000 | 0 | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300 | 0 | 10 | | 7936 | 23456 | BOSCO-DBA | 7788 | 2014-09-13 16:13:56 | 2450 | 800 | 10 | +-------+--------+-----------+------+---------------------+------+------+--------+ 15 rows in set (0.00 sec)
首先提个问题，如上测试表emp中empno是主键，类型为int，那么：
select * from emp where empno='7788';
会产生隐式转换吗？
下面实验证明：
MySQL [test]> select * from emp where empno=7788; +-------+-------+---------+------+---------------------+------+------+--------+ | 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) MySQL [test]> select * from emp where empno='7788'; +-------+-------+---------+------+---------------------+------+------+--------+ | 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)
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)
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;
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 | +----+-------------+-------+------+---------------+-----------+---------+-------+------+-----------------------+ 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
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