Note: mysqldate_formate

Note: mysqldate_formate

Today, I helped my colleagues handle an SQL statement (simplified) and reported the following error:

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 '>'

At first glance, it was quite inexplicable. I checked the manual and found that there was such a paragraph:

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.

That is to say, the result returned by the DATE_FORMATE () function carries the character set/verification set attribute, while the TIMEDIFF () function does not have the character set/verification set attribute. Let's verify it:

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                    |+--------------------------------------------+-----------------------------------------------+

As you can see, the character SET returned by the DATE_FORMAT () function varies with the character_set_connection and collation_connection values modified through set names. In this case, if you want to work properly, you need to convert the result into a character set. For example:

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 |+----------------------------------------------------------------------------------------------+

You can :)