MYSQL教程MySQL中DATE_FORMATE函数使用时的注意点

《MYSQL教程MySQL中DATE_FORMATE函数使用时的注意点》要点：
本文介绍了MYSQL教程MySQL中DATE_FORMATE函数使用时的注意点，希望对您有用。如果有疑问，可以联系我们。

MYSQL应用今天帮同事处理一个SQL(简化过后的)执行报错：

MYSQL应用
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应用乍一看挺莫名其妙的,查了下手册,发现有这么一段：

MYSQL应用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”).

MYSQL应用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应用也就是说,DATE_FORMATE() 函数返回的结果是带有字符集/校验集属性的,而 TIMEDIFF() 函数则没有字符集/校验集属性,我们来验证一下：

MYSQL应用
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','2013-11-20')) |
+--------------------------------------------+-----------------------------------------------+
| gb2312                   | binary                    |
+--------------------------------------------+-----------------------------------------------+

MYSQL应用可以看到,随着通过 SET NAMES 修改 character_set_connection、collation_connection? 值,DATE_FORMAT() 函数返回结果的字符集也跟着不一样.在这种情况下,想要正常工作,就需要将结果进行一次字符集转换,例如：

MYSQL应用
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','2013-11-20') using utf8) |
+----------------------------------------------------------------------------------------------+
|                                              1 |
+----------------------------------------------------------------------------------------------+

MYSQL应用就可以了 :)

（编辑：李大同）

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