MySQL中DATE_FORMATE函数内置字符集解析

今天在利用DATE_FORMATE处理两个日期判断时发现提示 Illegal mix of collations (utf8_general_ci,COERCIBLE),这个是字符集问题呀,但我是日期怎么会这样,当时没看到,后来看到一篇文章才想起来,下面我们一起来看看解决办法.

处理一个SQL,简化过后的执行报错,代码如下:

1. mysql> select date_format('2013-11-19','Y-m-d') > timediff('2013-11-19', '2013-11-20');                            
2.  
3. 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,代码如下:

1. mysql> set names utf8; 
2. mysql> select charset(date_format('2013-11-19','Y-m-d')), charset(timediff('2013-11-19', '2013-11-20')); 
3. +--------------------------------------------+-----------------------------------------------+ 
4. | charset(date_format('2013-11-19','Y-m-d')) | charset(timediff('2013-11-19', '2013-11-20')) | 
5. +--------------------------------------------+-----------------------------------------------+ 
6. | utf8                                       | binary                                        | 
7. +--------------------------------------------+-----------------------------------------------+ 
8.  
9. mysql> set names gb2312; 
10. mysql> select charset(date_format('2013-11-19','Y-m-d')), charset(timediff('2013-11-19', '2013-11-20')); 
11. +--------------------------------------------+-----------------------------------------------+ 
12. | charset(date_format('2013-11-19','Y-m-d')) | charset(timediff('2013-11-19', '2013-11-20')) | 
13. +--------------------------------------------+-----------------------------------------------+//开源代码Vevb.com 
14. | gb2312                                     | binary                                        | 
15. +--------------------------------------------+-----------------------------------------------+ 

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,代码如下:

1. mysql> select date_format('2013-11-19','Y-m-d') > convert(timediff('2013-11-19', '2013-11-20') using utf8); 
2. +----------------------------------------------------------------------------------------------+ 
3. | date_format('2013-11-19','Y-m-d') > convert(timediff('2013-11-19', '2013-11-20') using utf8) | 
4. +----------------------------------------------------------------------------------------------+ 
5. |                                                                                            1 | 
6. +----------------------------------------------------------------------------------------------+  

就可以了.

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

附后,下面是函数的参数说明: