SQL-联接3表查询

Question

Good day everyone,

This is my first post on stackoverflow. Honestly, the community is great and full of knowledge. I usually get on it but never made any posts until today I decided to register an account. I need the knowledge of this amazing community for a small quick join query.

For some odd reason I am not able to get it to work.

Here is the relationship table:

What I am trying to output (In this example I will use userID 10):

Display tweet from the Tweet table of all the user the userID 10 follows (including the userID 10 itself)

In other words: Assume userID 10 is called Bob and Bob follows Bell and Rogers.

So I'd like to output the tweet of Bob, and everyone Bob follows: Bell and Rogers

Thank you in advance for your help and time :)

EDIT:

My apologies! Here is the sample data:

Sample Data

Thank you once again :)

Answer 1

Without sample data it's hard to be certain but this query should work. It selects all tweets from the table which match a userID (1) or anyone who that user is following:

SELECT DISTINCT t.*
FROM User u
JOIN Follow f ON f.follower = u.userID
LEFT JOIN Tweet t ON t.userID = u.userID OR t.userID = f.following
WHERE u.userID = 1

Output (for your sample data)

tweetID     userID  tweet
1           1       Hello
2           4       Hey
3           1       Hi Folks!
6           4       Something
7           3       Heya!

Demo on dbfiddle

Answer 2

This will select tweet text of the user 10 and anyone he follows. Note that you need to put ID on 2 places.

SELECT t.userID, t.tweet FROM Tweet t
INNER JOIN
(SELECT following FROM Follow WHERE follower=10) uf
ON t.userID=uf.following
UNION
SELECT ut.userID, ut.tweet FROM Tweet ut
WHERE ut.userID=10

output (userID, tweet):

3   tweet2
2   tweet3
1   tweet1

Answer 3

This should work:

select t.tweet
from tweet t inner join  
                  (select b.userid,a.following
                   from follow a inner join users b
                   on a.follower = b.userid) u 
on t.userid = u.following
where u.userid = 1 or t.userid = 1
order by t.date,tweet