multiple LEFT JOIN using LIMIT on one of the joins - mysql

Question

I looked around for answers to this question but all the ones I tried simply didn't work. The other answer suggestions all threw errors for me. Maybe it's because I'm using using MariaDB ?.
SELECT * FROM 'view_winners'
I need top 3 in column 'class'
table view_winners is multiple left joins and I could not figure out how to limit 3 of the left join on table allClasses.
view_winners is:
`$view = "view_winners";
$db->query("DROP $view");
$db->query("CREATE VIEW $view AS
SELECT *
FROM thw22 evnt
LEFT JOIN allUsers usr
ON usr.user_id = evnt.e_owner_id
LEFT JOIN hw_vehicles veh
ON veh.vehicle_id = evnt.e_vehicle_id
LEFT JOIN hw_m_vehicle_class mcls
ON mcls.v_class_id = evnt.e_class_id
LEFT JOIN allClasses cls
ON mcls.cvm_id = cls.class_id
LEFT JOIN hw_v_scores sco
ON sco.v_score_id = evnt.e_score_id
WHERE (cls.class_name <> '' OR cls.class_name IS NOT NULL)
AND (sco.total <> '' OR sco.total IS NOT NULL)
ORDER BY cls.vehicle_type ASC, cls.class_name ASC, sco.total DESC
");`
It's probably best if I could LIMIT 3 on LEFT JOIN allClasses but I can't figure that out. So I figured I would loop through the result and unset rows over 3 in class in PHP. But again I could not figure out how to compare rows as looping through.
I need help with the LIMIT 3 on the JOIN or how to compare the results unsetting rows.
entry
class
score
786
sally
99
234
sally
90
456
bob
45
621
joe
90
964
joe
80
548
joe
66
346
joe
22
900
frank
89
700
frank
86
800
frank
72
123
frank
70
860
frank
50
333
frank
45
Desired results:
entry
class
score
786
sally
99
234
sally
90
456
bob
45
621
joe
90
964
joe
80
548
joe
66
900
frank
89
700
frank
86
800
frank
72

Answer 1

Might this answer help
And to clarify, it appears that for example, you want AT MOST, 3 entries per class (person name per sample data). If one class has only a single entry, get it. However, if someone else has 8 classes you want only the first 3 based on some pre-determined priority ordering, such as top 3 scores.
In your case, the OVER is partitioned by the "class", and the order by will be the score DESC (descending). So having the view give you this extra computed column (per class), you can then filter WHERE finalColumnNameYouAssign <= 3

Answer 2

I just answered similar question yesterday. See How to limit SQL query with JOIN
Here's my solution based on 2 linked tables, users and history for each user. There is also another solution there depending on your MySQL version.
SELECT *
FROM
`users` AS u
LEFT JOIN `history` AS h ON u.id = h.user_id
WHERE
FIND_IN_SET(h.id, (SELECT `list` FROM
(SELECT user_id, SUBSTRING_INDEX(GROUP_CONCAT(id SEPARATOR ','), ',', 3) AS `list` FROM
(SELECT h.user_id, h.id
FROM
`users` AS u
LEFT JOIN `history` AS h ON u.id = h.user_id
) AS `a`
GROUP BY user_id
) AS `b`
WHERE b.user_id = u.id
) )

Answer 3

Instead of having so many joins and confusing myself badly, I made a couple different views that I will need anyway for other statistics. Then by simply ordering by class ASC, score DESC I have a very simple master list of all classes in order then scores highest to lowest (along with all the other joined data). After that I can compare each row and limit 3 as follows:
SELECT * FROM
(SELECT *,
#rn := IF(#prev = class_name,
#rn + 1, 1)
AS rn,
#prev := class_name
FROM view_allScores
JOIN (SELECT #prev := NULL, #rn := 0)
AS vars
) AS T1
WHERE T1.rn <= 3
The confusing thing was that I was trying to add a LIMIT to a join and that kept confusing me.