默认情况下，MySQL.db表中包含的行表示任意用户可以访问test数据库和test_开头的数据库。这些行的User字段的值为空，表示匹配任意用户。这意味着这些数据库（test数据库和test_开头的数据库）默认可以被任意用户使用（即使没有权限的用户）。
表mysql.db的默认数据如下
mysql> select * from mysql.db\G
*************************** 1. row ***************************
Host: %
Db: test
User:
Select_priv: Y
Insert_priv: Y
Update_priv: Y
Delete_priv: Y
Create_priv: Y
Drop_priv: Y
Grant_priv: N
References_priv: Y
Index_priv: Y
Alter_priv: Y
Create_tmp_table_priv: Y
Lock_tables_priv: Y
Create_view_priv: Y
Show_view_priv: Y
Create_routine_priv: Y
Alter_routine_priv: N
Execute_priv: N
Event_priv: Y
Trigger_priv: Y
*************************** 2. row ***************************
Host: %
Db: test\_%
User:
Select_priv: Y
Insert_priv: Y
Update_priv: Y
Delete_priv: Y
Create_priv: Y
Drop_priv: Y
Grant_priv: N
References_priv: Y
Index_priv: Y
Alter_priv: Y
Create_tmp_table_priv: Y
Lock_tables_priv: Y
Create_view_priv: Y
Show_view_priv: Y
Create_routine_priv: Y
Alter_routine_priv: N
Execute_priv: N
Event_priv: Y
Trigger_priv: Y
2 rows in set (0.00 sec)
可以看到，任意用户对test数据库和test_开头的数据库都拥有很大的权限（上述为Y的权限）
下面验证上述权限
#创建一个只读账号
mysql> grant select on yujx.t to 'select'@'localhost' identified by 'select';
Query OK, 0 rows affected (0.00 sec)
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
#使用只读用户连接mysql
mysql> select user();
+------------------+
| user() |
+------------------+
| select@localhost |
+------------------+
1 row in set (0.00 sec)
mysql> show grants for 'select'@'localhost';
+---------------------------------------------------------------------------------------------------------
| Grants for select@localhost |
+---------------------------------------------------------------------------------------------------------
| GRANT USAGE ON *.* TO 'select'@'localhost' IDENTIFIED BY PASSWORD '*852200EDF18814F8BFC1F1DC816AAC4152D8262E'
| GRANT SELECT ON `yujx`.`t` TO 'select'@'localhost' |
+-------------------------------------------------------------------------------------------------
2 rows in set (0.00 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| test |
| test_a |
| yujx |
+--------------------+
4 rows in set (0.00 sec)
#操作test库
mysql> use test;
Database changed
#可以创建表
mysql> create table t(x int);
Query OK, 0 rows affected (0.01 sec)
#可以insert表
mysql> insert into t select 1;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
#可以drop database
mysql> drop database test;
Query OK, 1 row affected (0.01 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| test_a |
| yujx |
+--------------------+
3 rows in set (0.00 sec)
#同样适用于test_开头的库
mysql> use test_a
Database changed
mysql> create table a ( x int);
Query OK, 0 rows affected (0.01 sec)
mysql> show tables;
+------------------+
| Tables_in_test_a |
+------------------+
| a |
+------------------+
1 row in set (0.00 sec)
mysql> drop table a;
Query OK, 0 rows affected (0.01 sec)
mysql> drop database test_a;
Query OK, 0 rows affected (0.00 sec)
#创建数据库
#只要是dbname以test开头的都能创建成功
mysql> create database test;
Query OK, 1 row affected (0.00 sec)
mysql> create database test_a;
Query OK, 1 row affected (0.00 sec)
mysql> create database test_b;
Query OK, 1 row affected (0.00 sec)
mysql> create database a;
ERROR 1044 (42000): Access denied for user 'select'@'localhost' to database 'a'
#delete from mysql.db where db like 'test%'