如何在SQL中正确连接这四个表？

Question

I'm currently creating a small application, where users can subscribe to different channels to see the corresponding posts.

My database looks like this (simplified):

Now I'm trying to get all the information out of the post table (all columns), for all the posts that are connected to any channel a specific user may have subscribed.

Example: A user (id_user = 0) has subsribed to three channels (id_channel = 1, 2 and 3) and now wants to see all posts in these channels (each post where fk_channel = 1 or 2 or 3).

I tried many variations using different joins, however I couldn't manage to find the correct query. One of my last attempts looked something like this (which does not work...):

SELECT * FROM posts
INNER JOIN channels ON posts.fk_channel = channels.id_channel
INNER JOIN subscriptions ON channels.id_channel = subscriptions.fk_subcribed_channel
INNER JOIN users ON subscriptions.fk_subscibed_by = users.id_user

Answer 1

If you have the user's id then there is no need to join the table users , because you can use the column fk_subscibed_by of subscriptions :

SELECT p.* 
FROM subscriptions s
INNER JOIN channels c ON c.id_channel = s.fk_subcribed_channel
INNER JOIN posts p ON p.fk_channel = c.id_channel
WHERE s.fk_subscibed_by = 0

If you want to get all the columns of the joined tables, use SELECT * instead of SELECT p.* which returns only the columns from posts .

Answer 2

If your rappresentation is right this should work properly:

SELECT * FROM posts
LEFT OUTER JOIN channel ON posts.fk_channel = channel.id_channel
LEFT OUTER subscriptions ON channel.id_channel = subscriptions.fk_subcribed_channel
LEFT OUTER users ON subscriptions.fk_subscibed_by = users.id_user

Like this you should be able to get each post from each channel the user is subscribed.