MySQL中日期比较时遇到的编码问题解决办法

今天帮同事处理一个SQL(简化过后的)执行报错:


mysql> select date_format('2013-11-19','Y-m-d') > timediff('2013-11-19', '2013-11-20');                                        

ERROR 1267 (HY000): Illegal mix of collations (utf8_general_ci,COERCIBLE) and (latin1_swedish_ci,NUMERIC) for operation '>'


乍一看挺莫名其妙的,查了下手册,发现有这么一段:

The language used for day and month names and abbreviations is controlled by the value of the lc_time_names system variable (Section 9.7, “MySQL Server Locale Support”).

The DATE_FORMAT() returns a string with a character set and collation given by character_set_connection and collation_connection so that it can return month and weekday names containing non-ASCII characters.


也就是说,DATE_FORMATE() 函数返回的结果是带有字符集/校验集属性的,而 TIMEDIFF() 函数则没有字符集/校验集属性,我们来验证一下:

mysql> set names utf8;

mysql> select charset(date_format('2013-11-19','Y-m-d')), charset(timediff('2013-11-19', '2013-11-20'));

+--------------------------------------------+-----------------------------------------------+

| charset(date_format('2013-11-19','Y-m-d')) | charset(timediff('2013-11-19', '2013-11-20')) |

+--------------------------------------------+-----------------------------------------------+

| utf8                                       | binary                                        |

+--------------------------------------------+-----------------------------------------------+

mysql> set names gb2312; mysql> select charset(date_format('2013-11-19','Y-m-d')), charset(timediff('2013-11-19', '2013-11-20')); +--------------------------------------------+-----------------------------------------------+ | charset(date_format('2013-11-19','Y-m-d')) | charset(timediff('2013-11-19', '2013-11-20')) | +--------------------------------------------+-----------------------------------------------+ | gb2312                                     | binary                                        | +--------------------------------------------+-----------------------------------------------+


可以看到,随着通过 SET NAMES 修改 character_set_connection、collation_connection  值,DATE_FORMAT() 函数返回结果的字符集也跟着不一样。在这种情况下,想要正常工作,就需要将结果进行一次字符集转换,例如:

mysql> select date_format('2013-11-19','Y-m-d') > convert(timediff('2013-11-19', '2013-11-20') using utf8);

+----------------------------------------------------------------------------------------------+

| date_format('2013-11-19','Y-m-d') > convert(timediff('2013-11-19', '2013-11-20') using utf8) |

+----------------------------------------------------------------------------------------------+

|                                                                                            1 |

+----------------------------------------------------------------------------------------------+

就可以了

P.S,MySQL的版本:5.5.20-55-log Percona Server (GPL), Release rel24.1, Revision 217