LEFT JOIN中的COUNT返回重复值

Question

I've the following tables (example):

users :

id | user | photo   | joined | country
1  | Igor | abc.jpg | 2015   | Brazil
2  | John | cga.png | 2014   | USA
3  | Lucas| hes.jpg | 2016   | Japan

posts (see that there are two lines with author = Igor and ft = 2 and one line with author = Igor and ft = 3 and Igor have three posts):

id | author | content | date | ft (2 = photos and 3 = videos)
1  | Igor   | hi      | 2016 | 2
2  | Igor   | hello   | 2016 | 3
3  | John   | hehehe  | 2016 | 2
4  | Igor   | huhuhuh | 2016 | 2
5  | Lucas  | lol     | 2016 | 3

friendship (when status = 2 means that they are friends):

id | friend1 | friend2 | status
1  | Igor    | Lucas   | 2
2  | Lucas   | John    | 2
3  | John    | Igor    | 2

And I want to do a COUNT of posts with ft = 2 and a COUNT of friends ( status = 2 ) according to the currently logged user (Igor, in this case).

So, I do (assuming that the current user logged in is Igor ):

SELECT photo, joined, country, sum(CASE WHEN ft = 2 THEN 1 ELSE 0 END) AS numPhotos, sum(CASE WHEN ft = 3 THEN 1 ELSE 0 END) AS numVideos
FROM users
LEFT JOIN posts
ON users.user = posts.author
WHERE users.user = 'Igor'
GROUP BY users.user
LIMIT 1

And when I check on a foreach , the data is correct: numPhotos = 2 and numVideos = 1 .

But, I want to select too the number of friends, so, I do:

SELECT photo, joined, country, sum(CASE WHEN ft = 2 THEN 1 ELSE 0 END) AS numPhotos, sum(CASE WHEN ft = 3 THEN 1 ELSE 0 END) AS numVideos, count(friendship.status) AS numFriends
FROM users
LEFT JOIN posts
ON users.user = posts.author
LEFT JOIN friendship
ON (users.user = friend1 OR users.user = friend2) AND friendship.status = 2
WHERE users.user = 'Igor'
GROUP BY users.user
LIMIT 1

But, the output is: numPhotos = 4 , numVideos = 2 and numFriends = 6 .

In other words, he is duplicating all results but in numFriends he's taking the total of posts of Igor (3) and duplicating the value too. And if I change count(friendship.status) to sum(friendship.status) the output is: numPhotos = 4 , numVideos = 2 and numFriends = 18 (triples the numFriends ).

I tried too with count(distinct friendship.status) and the result is: numPhotos = 4 , numVideos = 2 and numFriends = 1 (duplicates the values again as well as return the wrong value 1 for numFriends that should be 2 knowing he has two friends).

So, how I can do this? (I'm using MySQL)

EDIT :

I changed the count(distinct friendship.status) to count(distinct friendship.id) and it worked to select the number of friends. But the rest of values ( numPhotos and numVideos ) continue duplicated.

I discovered that the problem is in ON (users.user = friend1 OR users.user = friend2) , because if I leave only ON (users.user = friend1) or ON (users.user = friend2) the output isn't duplicated. I tried too with ON 'Igor' IN (friend1, friend2) but the result is the same ( numPhotos and numVideos` continue duplicated).

Answer 1

I think the left join may be joining on a one-to-many relationship, which is causing inflated counts. Since you are only retrieving the counts for 1 user, I suggest using a subquery to retrieve the friendship counts (for retrieving the counts for multiple users, a derived table may be faster than a subquery):

SELECT 
    sum(ft = 2) AS numPhotos, 
    sum(ft = 3) AS numVideos,
    (select count(*) from friendships f
    where (friend1 = users.user 
    or friend2 = users.user)
    and status = 2) as friendship_count
FROM users
LEFT JOIN posts
ON users.user = posts.author
WHERE users.user = 'Igor'

Note that I removed the group by because users.user is already in the where clause, which means there is only 1 group.

Answer 2

Instead of count(distinct friendship.status) , try using count(distinct friendship.id) . That should give you the number of unique friends. Counting distinct statuses doesn't work because all the statuses will be 2 by definition, so there is only one distinct value.