SQL query that produces the following output? - mysql

Question

I am trying to write an SQL statement producing the below output.
I have the two following tables:
UserMovie
userID | movieID
-----------------
135 | k0jps
135 | p1zka
125 | v0t67
115 | opp2s
111 | xnwri
115 | kspdl
Follows
followerid | followingid
------------------------
122 | 135
192 | 111
125 | 240
120 | 125
45 | 111
I want to fetch the number of followers of each user who's userid is in the UserMovie Table, giving the following result:
Result
userid | followerCount
----------------------
135 | 1
125 | 1
115 | 0
111 | 2
The following statement gives me partially what i want:
SELECT followingid, count(*) as followerCount
FROM Follows
WHERE followingid in (SELECT DISTINCT userID FROM UserMovie)
GROUP BY followingid
The issue with the above query is that users with 0 followers do not appear in the results giving the following output:
userid | followerCount
----------------------
135 | 1
125 | 1
111 | 2
Any idea on how to do it?

Answer 1

Try this to include users with no follows:
SELECT UserId, Count(followerid) AS followerCount
FROM (SELECT DISTINCT userId FROM UserMovie ) m
LEFT JOIN Follows f
ON f.followingid = m.userID
GROUP BY UserId
Now it generates :
UserId followerCount
111 2
115 0
125 1
135 1

Answer 2

The following worked for me.
However I am getting NULLs instead of 0 for users with no followers
SELECT DISTINCT u.userid, t.followerCount
FROM UserMovie u
LEFT JOIN (
SELECT followingid, count(*) AS followerCount
FROM Follows
WHERE followingid in (SELECT DISTINCT userID FROM UserMovie)
GROUP BY followingid ) as t
on t.followingid = u.userid

Answer 3

How about a solution using CASE?
SELECT userId,
CASE
WHEN IFNULL(followerid, 0) = 0 THEN 0
ELSE count(*)
END
FROM UserMovie
LEFT JOIN Follows on followingid=userID
GROUP BY userId;
Seems to work fine in SQLite3, just replace IFNULL with ISNULL (if SQLServer) or any other equivalent. It's pretty similar to what you've done.

Answer 4

Here's one approach: get a distinct list of userID from UserMovie in an inline view (use either a GROUP BY or a DISTINCT keyword), and perform an "outer join" operation of that to the Followers table to find followers. Collapse the rows from that with a GROUP BY, and use an aggregate function to get a count of unique/distinct non-null values of userId from the Followers table.
For example:
SELECT u.userID
, COUNT(DISTINCT f.userID) AS cnt_followers
FROM ( SELECT m.userID
FROM UserMovie m
GROUP BY m.userID
) u
LEFT
JOIN Follows f
ON f.followingid = u.userID
GROUP BY u.userID
EDIT
There's an invalid column reference in the SELECT list, f.userID is not valid. That should be f.followerID.
When we fix that, the query returns:
userID cnt_followers
111 2
115 0
125 1
135 1
SQL Fiddle HERE http://sqlfiddle.com/#!9/de3e7/2
As long as we are counting "distinct" followerid (question doesn't give any guarantee that (followerID,followingID) is UNIQUE in Followers table), we could eliminate the inline view
SELECT u.userID
, COUNT(DISTINCT f.userID) AS cnt_followers
FROM UserMovie u
LEFT
JOIN Follows f
ON f.followingid = u.userID
GROUP BY u.userID