MySQL中DATE_FORMATE函數(shù)使用時(shí)的注意點(diǎn)

更新時(shí)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(shí)的注意點(diǎn),主要是針對(duì)其內(nèi)置的字符集使用時(shí)需要轉(zhuǎn)換而進(jìn)行說(shuō)明,需要的朋友可以參考下

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)：

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：

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：

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：

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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