SQL中用where子句实现的表连接怎么用inner join实现？

Question

RBAC(Role-Based Access Control,基于角色的访问控制)用5张表实现:

用户表 rbac_user(id, name)
角色表 rbac_role(id, name)
操作表 rbac_oper(id, name)
用户对应的角色表 rbac_relation_user_role(user_id, role_id)
角色对应的操作表 rbac_relation_role_oper(role_id, oper_id, value)

建表并插入数据:

CREATE TABLE `rbac_user` (
    `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    `name` varchar(255) NOT NULL DEFAULT '',
    PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8;

CREATE TABLE `rbac_role` (
    `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    `name` varchar(255) NOT NULL DEFAULT '',
    PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8;

CREATE TABLE `rbac_oper` (
    `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    `name` varchar(255) NOT NULL DEFAULT '',
    PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8;

CREATE TABLE `rbac_relation_user_role` (
    `user_id` int(10) unsigned NOT NULL,
    `role_id` int(10) unsigned NOT NULL,
    UNIQUE KEY (`user_id`, `role_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `rbac_relation_role_oper` (
    `role_id` int(10) unsigned NOT NULL,
    `oper_id` int(10) unsigned NOT NULL,
    `value`   int(10) unsigned NOT NULL,
    UNIQUE KEY (`role_id`, `oper_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO rbac_user VALUES(1,'张三丰');
INSERT INTO rbac_role VALUES(1,'管理员');
INSERT INTO rbac_oper VALUES(1,'foo');
INSERT INTO rbac_oper VALUES(2,'bar');
INSERT INTO rbac_relation_user_role VALUES(1,1);
INSERT INTO rbac_relation_role_oper VALUES(1,1,1);
INSERT INTO rbac_relation_role_oper VALUES(1,2,0);

用where子句进行表连接获取编号为1的用户具有的操作权限:

select rbac_oper.name, rbac_relation_role_oper.value
from rbac_user, rbac_role, rbac_oper, rbac_relation_user_role, rbac_relation_role_oper
where rbac_relation_user_role.user_id = rbac_user.id 
and   rbac_relation_user_role.role_id = rbac_role.id
and   rbac_relation_role_oper.role_id = rbac_role.id
and   rbac_relation_role_oper.oper_id = rbac_oper.id
and   rbac_user.id = 1
结果:
name  value
foo     1
bar     0

现在的问题是用inner join获得相同的结果该怎么写?
我这样写,会提示错误:

select * from 
rbac_relation_user_role 
    inner join rbac_user on rbac_relation_user_role.user_id = rbac_user.id
    inner join rbac_role on rbac_relation_user_role.role_id = rbac_role.id
    inner join rbac_role on rbac_relation_role_oper.role_id = rbac_role.id
    inner join rbac_oper on rbac_relation_role_oper.oper_id = rbac_oper.id
where rbac_user.id = 1
错误: #1066 - Not unique table/alias: 'rbac_role'

Answer 1

select rbac_oper.name, rbac_relation_role_oper.value
from rbac_user, rbac_role, rbac_oper, rbac_relation_user_role, rbac_relation_role_oper
where rbac_relation_user_role.user_id = rbac_user.id 
and   rbac_relation_user_role.role_id = rbac_role.id
and   rbac_relation_role_oper.role_id = rbac_role.id
and   rbac_relation_role_oper.oper_id = rbac_oper.id
and   rbac_user.id = 1

等价于下面的inner join(缩写join)写法

select rbac_oper.name, rbac_relation_role_oper.value 
from rbac_relation_user_role 
join rbac_user               on rbac_relation_user_role.user_id = rbac_user.id
join rbac_role               on rbac_relation_user_role.role_id = rbac_role.id
join rbac_relation_role_oper on rbac_relation_role_oper.role_id = rbac_role.id
join rbac_oper               on rbac_relation_role_oper.oper_id = rbac_oper.id
where rbac_user.id = 1

注意其中:
join rbac_relation_role_oper on rbac_relation_role_oper.role_id = rbac_role.id
不能写作:
join rbac_role on rbac_relation_role_oper.role_id = rbac_role.id
因为要保证所有需要关联的表都被join,同一个表不能join两次.