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 :)