How to calculate a level with an unlimited number of levels? - mysql

Question

Now I have that structure (very simplify):
promotion:
id | name | level1 | points1| level2 | points2 | client_id
1 | A | 10 | 12 | 20 | 15 | 1
client:
id | name | value
1 | john | 15
And that's how I calculate the level:
SELECT
name,
CASE
WHEN client.value >= promotion.level2 THEN promotion.points2
WHEN client.value >= promotion.level1 THEN promotion.points1
ELSE "None"
END as points
FROM promotion
JOIN client ON client.id = promotion.client_id
This is working well, but I would like to have such structure:
promotion:
id | name | client_id
1 | A | 1
level:
id | name | level | points | promotion_id
1 | level1 | 10 | 12 | 1
2 | level2 | 10 | 15 | 1
client:
id | name | value
1 | john | 15
But I don't have any idea how to use it in my query to get points...
SELECT
name,
CASE
???
END as points
FROM promotion
JOIN client ON client.id = promotion.client_id
LEFT JOIN level ON promotion.id= level.promotion_id

Answer 1

This query will give you the desired results. It uses a LEFT JOIN to find any rows in the level table which are lower than the client value, and then takes the MAX of those values, using COALESCE to set the value to None if there are no levels below the client's value:
SELECT p.name AS promotion,
c.name AS client,
COALESCE(MAX(l.points), 'None') AS points
FROM promotion p
JOIN client c ON c.id = p.client_id
LEFT JOIN level l ON l.promotion_id = p.id AND l.level < c.value
GROUP BY p.name, c.name
Output for your sample data:
promotion client points
A john 12
Demo on dbfiddle

Answer 2

I would use a correlated subquery:
select p.name,
(select l.points
from level l
where l.promotion_id = p.points and
l.level <= c.value
order by l.level desc
limit 1
) as points
from promotion p join
client c
on c.id = p.client_id;
The point of using a correlated subquery is to avoid an aggregation on the full data. With an index on level(promotion_id, level, points), this should have better performance.