MySQL: Greatest n per group with joins and conditions - mysql

Question

Table Structure
I have a table similar to the following:
venues
The following table describes a list of businesses
id name
50 Nando's
60 KFC
rewards
The table describes a number of rewards, the venue it corresponds to and the number of points needed to redeem the reward.
id venue_id name points
1 50 5% off 10
2 50 10% off 20
3 50 11% off 30
4 50 15% off 40
5 50 20% off 50
6 50 30% off 50
7 60 30% off 70
8 60 60% off 100
9 60 65% off 120
10 60 70% off 130
11 60 80% off 140
points_data
The table describes the number of points remaining a user has for each venue.
venue_id points_remaining
50 30
60 90
Note that this query is actually computed within SQL like so:
select * from (
select venue_id, (total_points - points_redeemed) as points_remaining
from (
select venue_id, sum(total_points) as total_points, sum(points_redeemed) as points_redeemed
from (
(
select venue_id, sum(points) as total_points, 0 as points_redeemed
from check_ins
group by venue_id
)
UNION
(
select venue_id, 0 as total_points, sum(points) as points_redeemed
from reward_redemptions rr
join rewards r on rr.reward_id = r.id
group by venue_id
)
) a
group by venue_id
) b
GROUP BY venue_id
) points_data
but for this question you can probably just ignore that massive query and assume the table is just called points_data.
Desired Output
I want to get a single query that gets:
The top 2 rewards the user is eligible for each venue
The lowest 2 rewards the user is not yet eligible for for each venue
So for the above data, the output would be:
id venue_id name points
2 50 10% off 20
3 50 11% off 30
4 50 15% off 40
5 50 20% off 50
7 60 30% off 70
8 60 60% off 100
9 60 65% off 120
What I got so far
The best solution I found so far is first getting the points_data, and then using code (i.e. PHP) to dynamically write the following:
(
select * from rewards
where venue_id = 50
and points > 30
ORDER BY points desc
LIMIT 2
)
union all
(
select * from rewards
where venue_id = 50
and points <= 30
ORDER BY points desc
LIMIT 2
)
UNION ALL
(
select * from rewards
where venue_id = 60
and points <= 90
ORDER BY points desc
LIMIT 2
)
UNION ALL
(
select * from rewards
where venue_id = 60
and points > 90
ORDER BY points desc
LIMIT 2
)
ORDER BY venue_id, points asc;
However, I feel the query can get a bit too long and in-efficient. For example, if a user has points in 400 venues, that is 800 sub-queries.
I tried also doing a join like so, but can't really get better than:
select * from points_data
INNER JOIN rewards on rewards.venue_id = points_data.venue_id
where points > points_remaining;
which is far from what I want.

Answer 1

Correlated subqueries counting the number of higher or lower rewards to determine the top or bottom entries are one way.
SELECT r1.*
FROM rewards r1
INNER JOIN points_data pd1
ON pd1.venue_id = r1.venue_id
WHERE r1.points <= pd1.points_remaining
AND (SELECT count(*)
FROM rewards r2
WHERE r2.venue_id = r1.venue_id
AND r2.points <= pd1.points_remaining
AND (r2.points > r1.points
OR r2.points = r1.points
AND r2.id > r1.id)) < 2
OR r1.points > pd1.points_remaining
AND (SELECT count(*)
FROM rewards r2
WHERE r2.venue_id = r1.venue_id
AND r2.points > pd1.points_remaining
AND (r2.points < r1.points
OR r2.points = r1.points
AND r2.id < r1.id)) < 2
ORDER BY r1.venue_id,
r1.points;
SQL Fiddle
Since MySQL 8.0 a solution using the row_number() window function would be an alternative. But I suppose you are on a lower version.
SELECT x.id,
x.venue_id,
x.name,
x.points
FROM (SELECT r.id,
r.venue_id,
r.name,
r.points,
pd.points_remaining,
row_number() OVER (PARTITION BY r.venue_id,
r.points <= pd.points_remaining
ORDER BY r.points DESC) rntop,
row_number() OVER (PARTITION BY r.venue_id,
r.points > pd.points_remaining
ORDER BY r.points ASC) rnbottom
FROM rewards r
INNER JOIN points_data pd
ON pd.venue_id = r.venue_id) x
WHERE x.points <= x.points_remaining
AND x.rntop <= 2
OR x.points > x.points_remaining
AND x.rnbottom <= 2
ORDER BY x.venue_id,
x.points;
db<>fiddle
The tricky part is here to partition the set also into the subset where the points of the user are enough to redeem the reward and the one where the points aren't enough, per venue. But as in MySQL logical expressions evaluate to 0 or 1 (in non Boolean context), the respective expressions can be used for that.