另一张表中的“WHERE”声明

Question

我有一个包含3个字段的关注/关注者表：

id , FollowingUserName,FollowedUserName

我有一张帖子的表格：

id,Post,PublishingUsername

我需要一个从post

返回某些字段的查询

但“where”将在哪里：

• 帖子中的PublishingUsernam将与FollowedUserName表格中的Following/Followers匹配
• FollowingUserName将登录用户名。

Answer 1

你正在寻找一个JOIN的样子。基本上，您希望从发布用户名=后跟用户名的帖子表中选择，以及followusername = loggedin name。

刚刚进行了攻击（因为我现在没有SQL服务器），但它可能看起来像：

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

Answer 2

如果没有JOIN，您可以：

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

Answer 3

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

Answer 4

要获得帖子：

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

或者您可以使用加入：

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

Answer 5

使用更清晰的加入语法

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