《Mysql學習MySQL中日期比較時遇到的編碼問題解決辦法》要點：
本文介紹了Mysql學習MySQL中日期比較時遇到的編碼問題解決辦法，希望對您有用。如果有疑問，可以聯系我們。
本日幫同事處理一個SQL(簡化過后的)執行報錯：
MYSQL入門
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 '>'MYSQL入門
乍一看挺莫名其妙的,查了下手冊,發現有這么一段：
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.MYSQL入門
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：
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??????????????????????????????????????? |
+--------------------------------------------+-----------------------------------------------+MYSQL入門
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,就必要將結果進行一次字符集轉換,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 217MYSQL入門
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