对mysql.user进行手工操作，比如添加新用户或更改用户的旧密码后，需用flush privileges刷新MySQL的系统权限相关表，否则会出现拒绝访问！
以root身份登录数据库，创建用户yangql
mysql> create user yangql identified by 'yangql';
Query OK, 0 rows affected (0.02 sec)
mysql> exit
Bye
测试连接。
[root@rac3 home]# mysql -uyangql -pyangql
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| test |
+--------------------+
2 rows in set (0.00 sec)
mysql> exit
Bye
以root用户登录，手工修改mysql.user表，这里演示更新用户的密码，对于插入新建用户操作类似！
[root@rac3 home]# mysql
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| latin |
| lyz |
| momo |
| mysql |
| test |
| test2 |
| yangdb |
| yangutf |
+--------------------+
9 rows in set (0.00 sec)
mysql> update mysql.user set password=password('yql') where user='yangql';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from mysql.user where user='yangql';
+------+--------+------------------+-------------+
| Host | User | Password | Select_priv |
+------+--------+------------------+-------------+
| % | yangql | 66d7dc1c3dfc53e6 | N |
+------+--------+------------------+-------------+
1 row in set (0.00 sec)
mysql> exit
Bye
没有使用flush privilges之前，可以使用老的密码进行登录。
[root@rac3 home]# mysql -uyangql -pyangql
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| test |
+--------------------+
2 rows in set (0.00 sec)
mysql> exit
Bye
执行刷新操作：
[root@rac3 home]# mysql
mysql> flush privilges;//刷新系统权限相关的表
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'privilges' at line 1
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
mysql>
再次使用旧密码登录
[root@rac3 ~]# mysql -uyangql -pyangql
ERROR 1045 (28000): Access denied for user 'yangql'@'localhost' (using password: YES)
再次使用新密码登录
[root@rac3 ~]# mysql -uyangql -pyql
Welcome to the MySQL monitor. Commands end with ; or \g.
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| test |
+--------------------+
2 rows in set (0.00 sec)
mysql> exit
Bye
[root@rac3 ~]#
还有一种方法，就是重新启动mysql服务器，来使新设置生效。