Limit selected results by unique selected IDs when using left joins - mysql

Question

I have a table users and some other tables like images and products
Table users:
user_id user_name
1 andrew
2 lutz
3 sophie
4 michael
5 peter
6 oscor
7 anton
8 billy
9 henry
10 jon
Tables images:
user_id img_type img_url
1 0 url1
1 1 url4
2 0 url5
7 0 url7
8 0 url8
9 1 url9
Table Products
user_id prod_id
1 5
1 55
2 555
8 5555
9 5
9 55
I use this kind of SELECT:
SELECT * FROM
(SELECT user.user_id,user.user_name, img.img_type, prod.prod_id FROM
users
LEFT JOIN images img ON img.user_id = users.user_id
LEFT JOIN products prod ON prod.user_id = users.user_id
WHERE user.user_id <= 5) AS users
ORDER BY user.user_id ASC
The result should be the following output. Due to performance improvements, I use ORDER BY and an inner select. If I put a LIMIT 5 within the inner or outer select, things won't work. MySQL will hard LIMIT the results to 5. However I need the LIMIT of 5 (pagination) found unique user_id results which would lead to 9 in this case.
Can I use maybe an if-statement to push an array with found user_id and break/finish up the select when the array consist of 5 UIDs? Or can I modify somehow the select?
user_id user_name img_type prod_id
1 andrew 0 5
1 andrew 1 5
1 andrew 0 55
1 andrew 1 55
2 lutz 0 5
2 lutz 0 55
3 sophie null null
4 michael null null
5 peter null null
results: 9

Answer 1

LIMIT 5 and user_id <= 5 do not necessarily give you the same results. One reason: There are multiple rows (after the JOINs) for user_id = 1. This is because there can be multiple images and/or multiple products for a given 'user'.
So, first decide which you want.
LIMIT without ORDER BY gives you an arbitrary set of rows. (Yeah, it is somewhat predictable, but you should not depend on it.)
ORDER BY + LIMIT usually implies gathering all the potentially relevant rows, sorting them, then doing the "limit". There are sometimes ways around this sluggishness.
LEFT leads to the NULLs you got; did you want that?
What do you want pagination to do if you are displaying 5 items per page, but user 1 has 6 images? You need to think about this edge case before we can help you with a solution. Maybe you want all of user 1 on a page, even if it exceeds 5? Maybe you want to break in the middle of '1'; but then we need an unambiguous way to know where to continue from for the next page.
Probably any viable solution will not use nested SELECTs. As you are finding out, it leads to "errors". Think of it this way: First find all the rows you need to display on all the pages, then carve out 5 for the current page.
Here are some more musings on pagination: http://mysql.rjweb.org/doc.php/pagination