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MySQL中DATE_FORMATE函數(shù)使用時的注意點

更新時間：2015年05月09日 10:23:05 投稿：goldensun

這篇文章主要介紹了MySQL中DATE_FORMATE函數(shù)使用時的注意點,主要是針對其內(nèi)置的字符集使用時需要轉(zhuǎn)換而進行說明,需要的朋友可以參考下

今天幫同事處理一個SQL（簡化過后的）執(zhí)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(fā)現(xiàn)有這么一段：

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.

也就是說，DATE_FORMATE() 函數(shù)返回的結(jié)果是帶有字符集/校驗集屬性的，而 TIMEDIFF() 函數(shù)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(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)果進行一次字符集轉(zhuǎn)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 :)

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