今天幫同事處理一個SQL（簡化過後的）執行報錯：
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，查了下手冊，發現有這麼一段：
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.
也就是說，DATE_FORMATE() 函數返回的結果是帶有字符集/校驗集屬性的，而 TIMEDIFF() 函數則沒有字符集/校驗集屬性，我們來驗證一下：
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，隨著通過 SET NAMES 修改 character_set_connection、collation_connection 值，DATE_FORMAT() 函數返回結果的字符集也跟著不一樣。在這種情況下，想要正常工作，就需要將結果進行一次字符集轉換，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 :)
