MySQL中DATE_FORMATE函数使用时的注意点
今天帮同事处理一个SQL（简化过后的）执行报错：
mysql>selectdate_format('2013-11-19','Y-m-d')>timediff('2013-11-19','2013-11-20');
ERROR1267(HY000):Illegalmixofcollations(utf8_general_ci,COERCIBLE)and(latin1_swedish_ci,NUMERIC)foroperation'>'
乍一看挺莫名其妙的，查了下手册，发现有这么一段：
Thelanguageusedfordayandmonthnamesandabbreviationsiscontrolledbythevalueofthelc_time_namessystemvariable(Section9.7,“MySQLServerLocaleSupport”).
TheDATE_FORMAT()returnsastringwithacharactersetandcollationgivenbycharacter_set_connectionandcollation_connectionsothatitcanreturnmonthandweekdaynamescontainingnon-ASCIIcharacters.
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>setnamesutf8;
mysql>selectcharset(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>setnamesgb2312;
mysql>selectcharset(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，随着通过SETNAMES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>selectdate_format('2013-11-19','Y-m-d')>convert(timediff('2013-11-19','2013-11-20')usingutf8);
+----------------------------------------------------------------------------------------------+
|date_format('2013-11-19','Y-m-d')>convert(timediff('2013-11-19','2013-11-20')usingutf8)|
+----------------------------------------------------------------------------------------------+
|1|
+----------------------------------------------------------------------------------------------+
就可以了:)