字符串连接

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mysql> select 'abc'+'123';+-------------+| 'abc'+'123' |+-------------+|         123 |+-------------+1 row in set, 1 warning (0.00 sec)mysql> select '123'+'123';+-------------+| '123'+'123' |+-------------+|         246 |+-------------+1 row in set (0.00 sec)mysql> select 123+123;+---------+| 123+123 |+---------+|     246 |+---------+1 row in set (0.00 sec)mysql> select concat('123','123');+---------------------+| concat('123','123') |+---------------------+| 123123              |+---------------------+1 row in set (0.00 sec)mysql> select concat('123',123);+-------------------+| concat('123',123) |+-------------------+| 123123            |+-------------------+1 row in set (0.00 sec)mysql> select concat(123,123);+-----------------+| concat(123,123) |+-----------------+| 123123          |+-----------------+1 row in set (0.00 sec)mysql> select concat(123,123,123);+---------------------+| concat(123,123,123) |+---------------------+| 123123123           |+---------------------+1 row in set (0.00 sec)

?仔细观察上面测试，MYSQL会尝试将加号两端的字段值尝试转换为数字类型，如果转换失败则认为字段值为0。在MYSQL中进行字符串的拼接要使用CONCAT函数，CONCAT函数支持一个或者多个参数，参数类型可以为字符串类型也可以是非字符串类型，对于非字符串类型的参数MYSQL将尝试将其转化为字符串类型，CONCAT函数会将所有参数按照参数的顺序拼接成一个字符串做为返回值。与MYSQL不同，MSSQLServer中可以直接使用加号“+”来拼接字符串。