根据文档:
character_set_database
...
被默认 数据库使用的字符集。只要默认数据库 更改,服务器就会设置此变量。如果没有默认数据库,则该变量与character_set_server具有相同的 值。
...
和
collation_database
...
由默认的数据库使用的排序规则。只要默认数据库发生更改, 服务器就会设置此变量。如果 没有默认数据库,则该变量具有与 collation_server相同的值。
...
与两个句子将获得相同的结果:
SELECT
default_character_set_name, default_collation_name
FROM
information_schema.SCHEMATA
WHERE
SCHEMA_NAME = SCHEMA()
和
select @@character_set_database, @@collation_database
在下面的测试证明:
mysql> DROP DATABASE IF EXISTS `my_database`;
Query OK, 0 rows affected (0.01 sec)
mysql> SELECT SCHEMA();
+----------+
| SCHEMA() |
+----------+
| NULL |
+----------+
1 row in set (0.00 sec)
mysql> SELECT
-> @@SESSION.character_set_database,
-> @@SESSION.collation_database,
-> @@SESSION.character_set_server,
-> @@SESSION.collation_server;
+----------------------------------+------------------------------+--------------------------------+----------------------------+
| @@SESSION.character_set_database | @@SESSION.collation_database | @@SESSION.character_set_server | @@SESSION.collation_server |
+----------------------------------+------------------------------+--------------------------------+----------------------------+
| latin1 | latin1_swedish_ci | latin1 | latin1_swedish_ci |
+----------------------------------+------------------------------+--------------------------------+----------------------------+
1 row in set (0.00 sec)
mysql> CREATE DATABASE IF NOT EXISTS `my_database`
-> CHARACTER SET utf8mb4
-> COLLATE utf8mb4_general_ci;
Query OK, 1 row affected (0.00 sec)
mysql> SELECT SCHEMA();
+----------+
| SCHEMA() |
+----------+
| NULL |
+----------+
1 row in set (0.00 sec)
mysql> SELECT
-> @@SESSION.character_set_database,
-> @@SESSION.collation_database,
-> @@SESSION.character_set_server,
-> @@SESSION.collation_server;
+----------------------------------+------------------------------+--------------------------------+----------------------------+
| @@SESSION.character_set_database | @@SESSION.collation_database | @@SESSION.character_set_server | @@SESSION.collation_server |
+----------------------------------+------------------------------+--------------------------------+----------------------------+
| latin1 | latin1_swedish_ci | latin1 | latin1_swedish_ci |
+----------------------------------+------------------------------+--------------------------------+----------------------------+
1 row in set (0.00 sec)
mysql> USE `my_database`;
Database changed
mysql> SELECT SCHEMA();
+-------------+
| SCHEMA() |
+-------------+
| my_database |
+-------------+
1 row in set (0.00 sec)
mysql> SELECT
-> @@SESSION.character_set_database,
-> @@SESSION.collation_database,
-> @@SESSION.character_set_server,
-> @@SESSION.collation_server;
+----------------------------------+------------------------------+--------------------------------+----------------------------+
| @@SESSION.character_set_database | @@SESSION.collation_database | @@SESSION.character_set_server | @@SESSION.collation_server |
+----------------------------------+------------------------------+--------------------------------+----------------------------+
| utf8mb4 | utf8mb4_general_ci | latin1 | latin1_swedish_ci |
+----------------------------------+------------------------------+--------------------------------+----------------------------+
1 row in set (0.00 sec)
mysql> SELECT
-> `DEFAULT_CHARACTER_SET_NAME`,
-> `DEFAULT_COLLATION_NAME`
-> FROM
-> `information_schema`.`SCHEMATA`
-> WHERE
-> SCHEMA_NAME = SCHEMA();
+----------------------------+------------------------+
| DEFAULT_CHARACTER_SET_NAME | DEFAULT_COLLATION_NAME |
+----------------------------+------------------------+
| utf8mb4 | utf8mb4_general_ci |
+----------------------------+------------------------+
1 row in set (0.00 sec)
然而,这两个句子的结合不会是错的:
mysql> USE `my_database`;
Database changed
mysql> SELECT
-> `DEFAULT_CHARACTER_SET_NAME`,
-> `DEFAULT_COLLATION_NAME`
-> FROM
-> `information_schema`.`SCHEMATA`
-> WHERE
-> SCHEMA_NAME = SCHEMA() AND
-> `DEFAULT_CHARACTER_SET_NAME` = @@SESSION.character_set_database AND
-> `DEFAULT_COLLATION_NAME` = @@SESSION.collation_database;
+----------------------------+------------------------+
| DEFAULT_CHARACTER_SET_NAME | DEFAULT_COLLATION_NAME |
+----------------------------+------------------------+
| utf8mb4 | utf8mb4_general_ci |
+----------------------------+------------------------+
1 row in set (0.00 sec)