查询用户：
    select User from user;
    
授权权限：
    授予指定权限：
    grant insert,select on *.* to ‘user_name‘@‘localhost‘ with grant option; (*.*的意思：如：A.B => 数据库A中的表B)
    授予所有权限：
    grant all privileges on *.* to ‘user_name‘@‘localhost‘ with grant option;
        
查看权限：
    show grants for ‘user_name‘@‘localhost‘;
    查看当前用户的权限：show grants;
    
回收权限：
    回收指定权限：
    revoke insert,select,grant option on *.* from ‘user_name‘@‘localhost‘ ...;
    回收所有权限：
    revoke all privileges,grant option from ‘user‘@‘localhost‘ ...;
    
    
    grant insert,select on *.* to ‘zhangsan‘@‘localhost‘ with grant option;
    show grants for ‘zhangsan‘@‘localhost‘;