今天幫同事處理一個(gè)SQL（簡(jiǎn)化過(guò)后的）執(zhí)行報(bào)錯(cuò)：

代碼如下:

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(cè)，發(fā)現(xiàn)有這么一段：

代碼如下:

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(shuō)，DATE_FORMATE() 函數(shù)返回的結(jié)果是帶有字符集/校驗(yàn)集屬性的，而 TIMEDIFF() 函數(shù)則沒(méi)有字符集/校驗(yàn)集屬性，我們來(lái)驗(yàn)證一下：

代碼如下:

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(guò) SET NAMES 修改 character_set_connection、collation_connection 值，DATE_FORMAT() 函數(shù)返回結(jié)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(jié)果進(jìn)行一次字符集轉(zhuǎn)換，例如：

代碼如下:

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

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