数据库多表查询的问题

Question

用户表有字段，

活动表有字段，

用户活动表有字段，

活动状态表有字段，
activity_status_name

需求，学生参加的活动。
获取的内容，学生姓名，活动姓名，活动状态。

select u.user_name,a.activity_name

from user u,activity a,activity_user au

where au.user_id = u.user_id

and au.activity_id = a.activity_id;

想增加活动状态时出错。

select u.user_name,a.activity_name ,ast.activity_status_name

from user u,activity a,activity_user au,activity_status ast

where au.user_id = u.user_id

and au.activity_id = a.activity_id

and ast.activity_status_id = '1';

求指教，是哪里的问题？基础较差，望理解。

Answer 1

缺少了一个连接条件。

select u.user_name,a.activity_name,ast.activity_status_name
from user u
join activity_user au on au.user_id = u.user_id
join activity a on a.activity_id = au.activity_id
join activity_status ast on a.activity_status_id = ast.activity_status_id;