“WHERE”语句从另一个表

Question

我有以下/追随者的表有3个字段：

id , FollowingUserName,FollowedUserName

而且我有个职位表：

id,Post,PublishingUsername 

我需要查询返回

的某些字段，但“where”将在哪里：

• 的PublishingUsernam从帖子将匹配FollowedUserName从Following/Followers表
• 和FollowingUserName将登录的用户名。

Answer 1

要获得帖子：

select p.* from posts p where p.PublishingUsername in 
(select FollowedUsername from followers) 
and p.PublishingUsername = LOGGEDINUSER 

或者你可以使用一个连接：

select p.* from posts p 
left join followers f on f.PublishingUsername = p.PublishingUsername 
and p.publishingUsername = LOGGENINUSER 

Answer 2

您正在寻找做JOIN它看起来像。基本上，您要从发布用户名后跟用户名的发布表中进行选择，以及followusername = loggedin name的位置。

只是走一个刺（因为我没有一个SQL服务器现在在这里），但它可能看起来像：

SELECT * FROM Posts INNER JOIN Following ON Posts.PublishingUsername = Following.FollowedUserName WHERE FollowingUserName = LoggedInName

Answer 3

没有JOIN您可以：

SELECT * FROM Posts p, Followers f 
WHERE 
    f.FollowingUserName = 'LoggedInUserName' 
    AND f.FollowedUserName = p.PublishingUsername 

Answer 4

SELECT 'fields'  
FROM TABLE1 INNER JOIN TABLE2 
ON TABLE1.FollowedUserName = TABLE2.PublishingUsername 
WHERE FollowingUserName = 'myvalue' 

Answer 5

使用清洁剂加入语法

SELECT * 
FROM following f 
JOIN posts p ON p.PublishingUsername = f.FollowedUsername 
AND f.FollowingUserName = LOGGED_IN;