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

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.