MySQL 隐式转换

 

字段类型定义

CREATE TABLE `user` (`id` int(10) NOT NULL AUTO_INCREMENT COMMENT '编号',/* ...... */`name` varchar(10) NOT NULL DEFAULT '' COMMENT '姓名',/* ...... */PRIMARY KEY (`id`),KEY `idx_name` (`name`,`nickname`),/* ...... */
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

 

索引类型注意

字段类型为“字符串”

• 查询：数字

mysql> EXPLAIN SELECT * FROM user WHERE name=123;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | user  | ALL  | idx_name      | NULL | NULL    | NULL | 21541 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)

• 查询：字符串

mysql> EXPLAIN SELECT * FROM user WHERE name='123';
+----+-------------+-------+------+---------------+----------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key      | key_len | ref   | rows | Extra                 |
+----+-------------+-------+------+---------------+----------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | user  | ref  | idx_name      | idx_name | 32      | const |    1 | Using index condition |
+----+-------------+-------+------+---------------+----------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)

对比结果

当字段类型是varchar等字符类时，如果查询值是数字类型，索引不生效。

 

字段类型为“数字”

• 查询：字符串

mysql> EXPLAIN SELECT * FROM user WHERE id='12';
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | user  | const | PRIMARY       | PRIMARY | 4       | const |    1 | NULL  |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

• 查询：数字

mysql> EXPLAIN SELECT * FROM user WHERE id=12;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | user  | const | PRIMARY       | PRIMARY | 4       | const |    1 | NULL  |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

对比结果

字符串会被转为整数处理，所以对定义为整数类型的字段而言，索引都可以用到；

说明

只有字段类型和查询的字段值类型对应时，才可用。

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隐式转换

mysql>  SELECT 'a'+123, 's'+'234', 5+'ab', 66+'kj', CONCAT(56, 'go');
+---------+-----------+--------+---------+------------------+
| 'a'+123 | 's'+'234' | 5+'ab' | 66+'kj' | CONCAT(56, 'go') |
+---------+-----------+--------+---------+------------------+
|     123 |       234 |      5 |      66 | 56go             |
+---------+-----------+--------+---------+------------------+
1 row in set, 4 warnings (0.00 sec)

说明

MySQL会将数字和字符串的直接相加，
转为整数型；可以使用CONCAT函数转为字符串类型；

隐式转化把字符串转为了整数类型。但是因为字符串是非数字型的，所以就会被转换为0

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MySQL文档说明

When an operator is used with operands of different types, type conversion occurs to make the operands compatible.当操作符与不同类型的操作数一起使用时，将进行类型转换以使操作数兼容。

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