Select specific titles based on friends' posts - php

Question

I have some tables.
titles
id| title
1 | Cars
2 | Computers
3 | Phones
4 | Tvs
entry
id | title_id | user_id | entry | time
1 | 1 | 12 | entry-01 | 1
2 | 2 | 11 | entry-02 | 2
3 | 3 | 12 | entry-03 | 3
4 | 2 | 11 | entry-04 | 4
5 | 3 | 11 | entry-05 | 5
6 | 4 | 12 | entry-06 | 6
7 | 4 | 13 | entry-07 | 7
8 | 4 | 11 | entry-08 | 8
9 | 1 | 10 | entry-09 | 9
10 | 2 | 12 | entry-10 | 10
users
id | username
10 | user-1
11 | user-2
12 | user-3
13 | user-4
friends
id | user_id | friend_id
1 | 10 | 12
2 | 11 | 12
3 | 12 | 10
4 | 10 | 11
I need to filter titles based on friends' entries and sort the results by (entry.time) desc. And I also need to show friends name and count(entry) at the list.
Expected result filtered by user_id=10 is:
result
1 | Computers | user-3, user-2(2)
2 | Tvs | user-2, user-3
3 | Phones | user-2, user-3
4 | Cars | user-3
any ideas?

Answer 1

This problem is complex, but if you get into the habit of breaking problems down into smaller pieces you will catch on pretty quickly. Why not start by getting all friends of user id 10? We can do so like this:
SELECT CASE WHEN user_id = 10 THEN friend_id
WHEN friend_id = 10 THEN user_id END AS userFriends
FROM friends
GROUP BY userFriends
HAVING userFriends IS NOT NULL;
Notice the use of a case statement, because user_id 10 could be in either of the two columns. I use the GROUP BY in case the user/friend pair appears multiple times (like 10 and 12 for your example) and a check for not null to remove the rows that didn't match the case.
Now that you have those, you can join it with the entries and titles tables to get the information you're going to need. Just add in some aggregation to get the number of entries each user has for a title:
SELECT t.title, u.userName, COUNT(*) AS numEntries
FROM titles t
LEFT JOIN entry e ON e.title_id = t.id
JOIN users u ON u.id = e.user_id
JOIN(
SELECT
CASE WHEN user_id = 10 THEN friend_id
WHEN friend_id = 10 THEN user_id END AS userFriends
FROM friends
GROUP BY userFriends
HAVING userFriends IS NOT NULL) f ON f.userFriends = u.id
GROUP BY t.title, u.userName;
Matching your format is going to be very tricky. Typically, you can use GROUP_CONCAT() to get a comma separated list, but you will get something like user3, user2, user2 for your first list. To fix this, I recommend writing a CONCAT() in your select statement to modify the above query to get the number of entries to the side of each user. In addition, use another CASE statement so that this only happens when the COUNT(*) is greater than 1:
SELECT t.title,
CASE WHEN COUNT(*) > 1 THEN
CONCAT(u.userName, ' (', COUNT(*), ')')
ELSE
u.userName
END AS numEntries
FROM titles t
LEFT JOIN entry e ON e.title_id = t.id
JOIN users u ON u.id = e.user_id
JOIN(
SELECT
CASE WHEN user_id = 10 THEN friend_id
WHEN friend_id = 10 THEN user_id END AS userFriends
FROM friends
GROUP BY userFriends
HAVING userFriends IS NOT NULL) f ON f.userFriends = u.id
GROUP BY t.title, u.userName;
And now, I would preform a GROUP_CONCAT() on that query:
SELECT tmp.title, GROUP_CONCAT(tmp.userEntries) AS friendEntries
FROM(
SELECT t.title,
CASE WHEN COUNT(*) > 1 THEN
CONCAT(u.userName, ' (', COUNT(*), ')')
ELSE
u.userName
END AS userEntries
FROM titles t
LEFT JOIN entry e ON e.title_id = t.id
JOIN users u ON u.id = e.user_id
JOIN(
SELECT
CASE WHEN user_id = 10 THEN friend_id
WHEN friend_id = 10 THEN user_id END AS userFriends
FROM friends
GROUP BY userFriends
HAVING userFriends IS NOT NULL) f ON f.userFriends = u.id
GROUP BY t.title, u.userName) tmp
GROUP BY tmp.title;
I apologize for the lengthy response (though I wanted to be clear and cover it all). If you've made it to this point, you'll be happy to know that it works in SQL Fiddle.