Mysql comparison of curtime() with a constant in utf8 - mysql

Question

I don't understand why the following comparison don't work in utf8 (mysql 5.5.27 - dotdeb).
select '00:00:00' < curtime();
Can you explain how mysql evaluates this expression because I don't see any logical reasons why this is not working :
mysql> set names 'utf8';
Query OK, 0 rows affected (0.00 sec)
mysql> select '00:00:00' < curtime();
ERROR 1267 (HY000): Illegal mix of collations (utf8_general_ci,COERCIBLE) and (latin1_swedish_ci,NUMERIC) for operation '<'
mysql> set names 'latin1';
Query OK, 0 rows affected (0.00 sec)
mysql> select '00:00:00' < curtime();
+------------------------+
| '00:00:00' < curtime() |
+------------------------+
| 1 |
+------------------------+
1 row in set (0.00 sec
An other example, it works if the comparison is done in a certain order :
mysql> set names 'utf8';
Query OK, 0 rows affected (0.00 sec)
mysql> select '00:00:00' < curtime();
ERROR 1267 (HY000): Illegal mix of collations (utf8_general_ci,COERCIBLE) and (latin1_swedish_ci,NUMERIC) for operation '<'
mysql> select curtime() > '00:00:00';
+------------------------+
| curtime() > '00:00:00' |
+------------------------+
| 1 |
+------------------------+
1 row in set (0.00 sec)
The collation and character variables in utf8 :
mysql> show variables like 'collation%';
+----------------------+-------------------+
| Variable_name | Value |
+----------------------+-------------------+
| collation_connection | utf8_general_ci |
| collation_database | latin1_swedish_ci |
| collation_server | latin1_swedish_ci |
+----------------------+-------------------+
3 rows in set (0.00 sec)
mysql> show variables like 'character%';
+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | utf8 |
| character_set_connection | utf8 |
| character_set_database | latin1 |
| character_set_filesystem | binary |
| character_set_results | utf8 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.00 sec)
This is working with MariaDb (5.5.25-MariaDB)
MariaDB [(none)]> set names 'utf8';
Query OK, 0 rows affected (0.00 sec)
MariaDB [(none)]> select '00:00:00' < curtime();
+------------------------+
| '00:00:00' < curtime() |
+------------------------+
| 1 |
+------------------------+
1 row in set (0.00 sec)
MariaDB [(none)]> SHOW VARIABLES LIKE 'character_set%';
+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | utf8 |
| character_set_connection | utf8 |
| character_set_database | latin1 |
| character_set_filesystem | binary |
| character_set_results | utf8 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.00 sec)
MariaDB [(none)]> SHOW VARIABLES LIKE 'collation%';
+----------------------+-------------------+
| Variable_name | Value |
+----------------------+-------------------+
| collation_connection | utf8_general_ci |
| collation_database | latin1_swedish_ci |
| collation_server | latin1_swedish_ci |
+----------------------+-------------------+
3 rows in set (0.00 sec)

Answer 1

You are trying to compare different data types - string and time, in this case MySQL uses type conversion and throws an error. Try to use STR_TO_DATE function to convert string to TIME and compare two identical types:
SET NAMES 'utf8';
SELECT STR_TO_DATE('00:00:00', '%H:%i:%s') < CURTIME();
More information here - Type Conversion in Expression Evaluation.

Answer 2

Check this solution, its working.
You need to cast value to time.
Eg 1 -
select cast('00:00:00' as time) < curtime();
Eg 2 -
CURTIME()<= ADDTIME(time_column,'-00:15:00')