MYSQL SUM() in one table and JOIN with 2 other tables - mysql

Question

I have 3 tables: items, purchases, and collaborators. A user can own an item, purchase an item, or be a collaborator on an item. Additionally, items that are purchased can be rated up, +1, or down, -1. An owner or collaborator can't purchase their own item.
I'd like to get all items for a given user and also display the ratings on each item.
Here's my tables:
items | purchases | collaborators
i_id item_id user_id | p_id item_id user_id rating |c_id item_id user_id
1 1 11 | 1 1 13 -1 | 1 1 12
2 2 12 | 2 2 11 1 | 2 2 13
3 3 13 | 3 3 12 NULL |
| 4 1 14 -1 |
Here's my MYSQL query so far:
select *, count(p_id) as tots, sum(rating=1) as yes, sum(rating= '-1') as no
from items
left join purchases
on items.item_id=purchases.item_id
left join collaborators
on items.item_id=collaborators.item_id
where items.user_id=13 or purchases.user_id=13 or collaborators.user_id=13
group by items.item_id
Here's my expected results for user_id=11 (changing each user_id in the WHERE clause):
item_id tots yes no
1 2 0 2
2 1 1 0
// notice how user_id=11 doesn't have anything to do with item_id=3
Here's my expected results for user_id=12:
item_id tots yes no
1 2 0 2
2 1 1 0
3 1 1 0
Here's my expected results for user_id=13:
item_id tots yes no
1 2 0 2
2 1 1 0
3 1 1 0
//notice user_id=13 should have same results as user_id=12. Although, their
relation to each of the 3 items is different, they still either purchased,
own, or collaboratored on each of them.
Unfortunately, I get the first two results but not the correct one for user_id=13.
For user_id=13, item_id=1 the tots=1 and not tots=2 for some reason I can't understand.
Any thoughts, such as, "its better to separate this into 2 queries", would be greatly appreciated,

Answer 1

I'm still not entirly sure I understand you correct but you could try following statement and let us work from there.
Edit
Following statement returns the expected results.
You can verify this (using SQL Server) here.
The gist of this is to
select all possible user_id and item_id combinations from your three tables
select the counts/ratings for each item
combine the results
SQL Statement
SELECT u.user_id, pt.item_id, pt.cnt, pt.yes, pt.no
FROM (
SELECT user_id, item_id, title FROM items
UNION SELECT user_id, item_id, NULL FROM purchases
UNION SELECT user_id, item_id, NULL FROM collaborators
) u INNER JOIN (
SELECT COUNT(*) AS cnt
, SUM(CASE WHEN ISNULL(rating, 1) = 1 THEN 1 ELSE 0 END) AS yes
, SUM(CASE WHEN rating =-1 THEN 1 ELSE 0 END) AS no
, item_id
FROM purchases
GROUP BY
item_id
) pt ON pt.item_id = u.item_id
MYSQL statement
SELECT u.user_id, pt.item_id, pt.cnt, pt.yes, pt.no, u.title
FROM (
SELECT user_id, item_id, title FROM items where user_id=13
UNION SELECT user_id, item_id, NULL FROM purchases where user_id=13
UNION SELECT user_id, item_id, NULL FROM collaborators where user_id=13
) u INNER JOIN (
SELECT COUNT(*) AS cnt
, SUM(rating=1) AS yes
, SUM(rating =-1) AS no
, item_id
FROM purchases
GROUP BY
item_id
) pt ON pt.item_id = u.item_id