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MySQL数据库:SQL语句中不可省去的引号

 

有一次在代码中发现了一个BUG,在排查过程中,差点晕翻;作一个DBA也碰到这种低级错误;大为自嘲。其实这个问题在官方手册里就写得很明白;(可见官方文档的重要性)

 

mysql >   create   table  h1 (id  int  , col1  char ( 1 ));Query OK,  0  rows affected ( 0.00  sec)mysql >   insert   into  h1  select   1 ,’I ' ;Query OK, 1 row affected (0.00 sec)Records: 1  Duplicates: 0  Warnings: 0mysql> insert into h1 select 2,’F ' ;Query OK,  1  row affected ( 0.00  sec)Records:  1   Duplicates:  0   Warnings:  0 mysql >   insert   into  h1  select   2 ,’ 0 ′;Query OK,  1  row affected ( 0.00  sec)Records:  1   Duplicates:  0   Warnings:  0mysql >   select   *   from  h1 ;+ —— + —— + |  id    |  col1  | + —— + —— + |      1   |  I     | |      2   |  F     | |      2   |   0      | + —— + —— + 3  rows  in   set  ( 0.00  sec)mysql >   select   *   from  h1  where  col1 = 0 ;+ —— + —— + |  id    |  col1  | + —— + —— + |      1   |  I     | |      2   |  F     | |      2   |   0      | + —— + —— + 3  rows  in   set ,  2  warnings ( 0.00  sec)mysql >   select   *   from  h1  where  col1 = ’ 0 ′;+ —— + —— + |  id    |  col1  | + —— + —— + |      2   |   0      | + —— + —— + 1  row  in   set  ( 0.00  sec)mysql >   select   *   from  h1  where  col1 = ’ 1 ′;Empty  set  ( 0.00  sec)mysql >   select   *   from  h1  where  col1 = 1 ;Empty  set ,  2  warnings ( 0.00  sec)

 

  发现没有:

 

  col1=0 ; 所有行被选中;

 

  col1=’0′ ; 结果集正常

 

mysql >   select   *   from  h1  where  col1 = ’ 1 ′;Empty  set  ( 0.00  sec)mysql >   select   *   from  h1  where  col1 = 1 ;Empty  set ,  2  warnings ( 0.00  sec)

 

  想知道为什么吗?

 

  看这里.

 

mysql >  show warnings;+ ——— + —— + ————————————— + |   Level     |  Code  |  Message                                | + ——— + —— + ————————————— + |  Warning  |   1292   |  Truncated incorrect  DOUBLE  value: ‘I’  | |  Warning  |   1292   |  Truncated incorrect  DOUBLE  value: ‘F’  | + ——— + —— + ————————————— + 2  rows  in   set  ( 0.00  sec)

 

  MySQL当数据类型不匹配时,尊重用户的输入,将字段的类型做了隐式转换然后来匹配,而字符向数字转,结果都是0;

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