Count distinct rows via a pair of known values - sql

Question

I wasn't even sure how to phrase this question. I'll give example content and wanted output, I'm looking for a query to do this.
Let's say I have table called "flagged" with this content:
content_id | user_id
1 | 1
1 | 2
1 | 3
2 | 1
2 | 3
2 | 4
3 | 2
3 | 3
4 | 1
4 | 2
5 | 1
6 | 1
6 | 4
And I have a a-symmetrical relationship between content_ids:
master_content_id | slave_content_id
1 | 2
3 | 4
5 | 6
For each "master" content_id (1, 3 and 5), I want to count how many distinct users have flagged either the master or the slave content, but count someone who flagged both as a single flag - which means that in the above example, content_id=1 was counted by user_id=1 (as content_id=1 and content_id=2), by user_id=2 (as content_id=1), by user_id=3 (as content_id=1 and content_id=2), and by user_id=4 (as content_id=2!)
An example of the output of the query I want to make is:
content_id | user_count
1 | 4 # users 1, 2, 3, 4
3 | 3 # users 1, 2, 3
5 | 2 # users 1, 4
I can't assume that the related content_ids are always a consecutive odd/even (i.e. 66 can be the master of the slave 58)
I am using MySQL and don't mind using its extensions to SQL (but rather the query be ANSI, or at least portable to the most databases)

Answer 1

The query below worked for me.
I'm using a sub-query with a UNION ALL to treat your mapped contents equal to the direct contents.
SELECT master_content_id AS content_id,
COUNT(DISTINCT user_id) AS user_count
FROM (
SELECT master_content_id, slave_content_id
FROM relationship
UNION ALL
SELECT master_content_id, master_content_id
FROM relationship
) r
JOIN flagged f ON ( f.content_id = r.slave_content_id )
GROUP BY master_content_id
Result:
content_id user_count
1 4
3 3
5 2

Answer 2

I think something like this will work for you (although GROUP_CONCAT is MySQL specific, similar concatenation can be achieved in other RDBMS)
SELECT COALESCE(Master_Content_ID, Content_ID) AS Content_ID,
COUNT(DISTINCT User_ID) AS Users,
CONCAT('#Users ', GROUP_CONCAT(DISTINCT User_ID ORDER BY User_ID)) AS UserList
FROM Flagged
LEFT JOIN MasterContent
ON Content_ID = Slave_Content_ID
GROUP BY COALESCE(Master_Content_ID, Content_ID)
Sample SQL Fiddle here: http://www.sqlfiddle.com/#!2/d09be/2
Output:
CONTENT_ID USERS USERLIST
1 4 #Users 1,2,3,4
3 3 #Users 1,2,3
5 2 #Users 1,4

Answer 3

From the samples given, does this do the job (I don't have MySQL available to test)?
SELECT
ms.master_content_id,
(SELECT COUNT(DISTINCT f.user_id) FROM flagged f WHERE
f.content_id = ms.slave_content_id OR
f.content_id = ms.master_content_id)
FROM
master_slave ms
It would be better not to have the DISTINCT, but I can't see a way around it.

Answer 4

SELECT master_content_id AS content_id
, COUNT(*) AS user_count
, GROUP_CONCAT(user_id) AS flagging_users
FROM
( SELECT r.master_content_id
, f.user_id
FROM relationship AS r
JOIN flagged AS f
ON f.content_id = r.master_content_id
UNION
SELECT r.master_content_id
, f.user_id
FROM relationship AS r
JOIN flagged AS f
ON f.content_id = r.slave_content_id
) AS un
GROUP BY master_content_id