Question

So I have 4 tables: users, posts, private, threads. In this example, lizzy created 2 private posts in different threads:

‘Dating’ post is for users 2,5,6 and herself to see the correct count of posts in that thread.

‘Break Ups’ post is for user 2 only and herself to see the correct count of posts in that thread.

Displaying the correct count depending on the user viewing the thread is the issue I am having. Here, we are focusing on lizzy, her threads, and post counts:

users                     (These aren't part of table. Just shows the counts we should display with our query depending on the user_id)
user_id |  user_name      //thread #: post_count-post_count_if_not_authorized = count to show 
--------------------
   1    |    tony         //thread 2: 3-1= 2 posts. thread 3: 2-1= 1 post.
   2    |    steph        //thread 2: 3-0= 3 posts. thread 3: 2-0= 2 posts.
   3    |    lizzy        //thread 2: 3 posts. thread 3: 2 posts.
   4    |    adam         //thread 2: 3-1= 2 posts. thread 3: 2-1= 1 post.
   5    |    lara         //thread 2: 3-0= 3 posts. thread 3: 2-1= 1 post.
   6    |    alexa        //thread 2: 3-0= 3 posts. thread 3: 2-1= 1 post.


posts
post_id   thread_id   user_id   post_name   private (0 is public, 1 is private to authorized users)
-----------------------------------------------------
   1         1           1       Coding        0
   2         2           3       Dating        1
   3         2           3       Show Me       0
   4         2           3       See Me        0 
   5         3           3       Break Ups     1
   6         3           3       True Love     0

private
private_id   post_id   authorized_user_id
-----------------------------------------------
    1           2               2
    2           2               5
    3           2               6
    4           5               2

threads
thread_id  user_id  post_count
------------------------------------
    1         1         1
    2         3         3  | When outputted in php, we should subtract the correct COUNT
    3         3         2  | from this depending on the user viewing the thread like above.

So basically, we have a total thread count with all posts in that thread. But if we pull that out with a mysql query, all users will see all lizzy’s post_count for each thread she has, when instead, only lizzy and any users she authorized to view certain posts on a thread should see the correct visible non private count for them. What would be the most efficient way to pull out the counts as a row (post_count_if_not_authorized) so we can subtract it from the post_count to show each user the correct count for them only?

Something like the below is what I am after (not working of course as is):

SELECT DISTINCT t.thread_id, t.post_count, t.COUNT(*)
FROM threads as t
JOIN posts as p on p.user_id = t.user_id
LEFT JOIN private pv on pv.post_id = p.post_id
WHERE t.user_id='3'
    AND (p.private = 0) OR (pv.authorized_user_id = {$logged_in_id} and p.private = 1)

UPDATE:

(t.user_id=’3′ in the WHERE clause is for lizzy in this example, and if $logged_in_id should give the correct count depending on the user, like the counts in the user table above)

Here is a fiddle.

The output should look like this if tony ($logged_in_id=1) is viewing lizzy’s (user_id=3) started threads:

thread_id    post_count_final (this should have the correct count for this user($logged_in_id) to see, all posts by lizzy - private posts by lizzy that this user is not authorized to see)
    2            2
    3            1

If steph ($logged_in_id=2) is viewing lizzy’s (user_id=3) started threads:

thread_id    post_count_final (this should have the correct count for this user($logged_in_id) to see, all posts by lizzy - private posts by lizzy that this user is not authorized to see)
    2            3
    3            2

(Note: The top right part next to the users table shows how these numbers are derived.)

Answer 1

SELECT 
t.thread_id, t.post_count, 
COUNT(IF(ISNULL(pv.private_id) AND p.private='1' AND p.user_id<>'1', null, 1)) 
FROM threads as t
JOIN posts as p on p.thread_id = t.thread_id 
JOIN users as u on u.user_id = p.user_id 
LEFT JOIN private as pv on pv.post_id = p.post_id AND pv.authorized_user_id='1' 
JOIN users as auth on auth.user_id = '1'
WHERE p.user_id='3' AND t.user_id='3'
GROUP BY t.thread_id;

This should work even if lizzy (logged in) is viewing lizzy. Adding COUNT(*) would return the same value as t.post_count. You could completely eliminate the use of threads table and do the counting in query all together, although the query will be more heavy.