A solution to the coding problem encountered during date comparisons in MySQL

Today, I helped my colleague to deal with an SQL (simplified) execution error report:

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 seems quite puzzling. After checking the manual, I found this 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.

In other words, the DATE_FORMATE() function returns a result with character set/check set attribute, while the TIMEDIFF() function does not have character set/check set attribute. Let's verify 1:

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 DATE_FORMAT() function returns a different character set as the character_set_connection and collation_connection values are modified through SET NAMES. In this case, to work properly, the result needs to be converted into a character set once, 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 |

+----------------------------------------------------------------------------------------------+

It is ok

P. S, MySQL versions: 5.5.20-55-ES31en Percona Server (GPL), Release rel24.1, Revision 217