mysql - curtime() と utf8 の定数との Mysql の比較

Question

次の比較が utf8 (mysql 5.5.27 - dotdeb) で機能しない理由がわかりません。

select '00:00:00' < curtime();

これが機能しない論理的な理由が見当たらないため、mysql がこの式をどのように評価するか説明してもらえますか :

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

別の例として、比較が特定の順序で行われる場合に機能します。

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)

utf8 の照合変数と文字変数:

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)

これはMariaDb（5.5.25-MariaDB）で動作しています

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)