也议MySQL中隐式转换

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目录(?)[+]

1. 环境说明

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RHEL 6.4 x86_64 + MySQL 5.6.19

测试表：

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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)

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</pre><pre name="code" class="sql">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)

2. 数值类型（int）

首先提个问题，如上测试表emp中empno是主键，类型为int，那么：

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select * from emp where empno='7788';

会产生隐式转换吗？

下面实验证明：

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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)

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：

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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：

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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：

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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)

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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。

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