sql - 来自另一个表的“WHERE”语句

Question

我有一个包含 3 个字段的关注者/关注者表：

id , FollowingUserName,FollowedUserName

我有一张有帖子的桌子：

id,Post,PublishingUsername

我需要一个从帖子返回某些字段的查询

但“哪里”将是哪里：

• From The PublishingUsernamPosts 将匹配FollowedUserNameFrom The Following/FollowersTable
• 并且FollowingUserName将是登录的用户名。

Answer 1

使用更简洁的 Join 语法

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

Answer 2

没有JOIN你可以：

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

只获取帖子：

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

或者您可以使用联接：

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

Answer 5

您正在寻找看起来像的JOIN 。基本上，您想从发布用户名 = 关注的用户名和关注用户名 = 登录名的帖子表中进行选择。

只是尝试一下（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