Calculating progressive pricing in PostgreSQL - sql

Question

I need to calculate revenue based on how many items a user has.
So for example, the first 10 items are free, up to 100 items are 0.50, up to 200 are 0.25 and up to 500 are 0.15 for example.
I have no idea where to start with this, can I get some direction please?
EG. If a user has 365 items, this would be (10 * 0) + (90 * 0.5) + (100 * 0.25) + (165 * 0.15)
Ideally I'd be doing this in python or something, but the dashboarding tool doesn't have that capability...
EDIT:
I should have mentioned that the number of items isn't actually the number they have, it's the limit they have chosen. The limit is saved as a single number in a subscription event. So for each user I will have an integer representing their max items eg. 365

Answer 1

First number items using window function row_number,
then use a case expression to assign a proper value for each item.
Simple example: http://sqlfiddle.com/#!17/32e4a/9
SELECT user_id,
SUM(
CASE
WHEN rn <= 10 THEN 0
WHEN rn <= 100 THEN 0.5
WHEN rn <= 200 THEN 0.25
WHEN rn <= 500 THEN 0.15
ELSE 0.05
END
) As revenue
FROM (
SELECT *,
row_number() OVER (partition by user_id order by item_no ) As rn
FROM mytable
) x
GROUP BY user_id
I should have mentioned that the number of items isn't actually the
number they have, it's the limit they have chosen. The limit is saved
as a single number in a subscription event. So for each user I will
have an integer representing their max items eg. 365
In this case the below query probably fits your needs:
Demo: http://sqlfiddle.com/#!17/e7a6a/2
SELECT *,
(SELECT SUM(
CASE
WHEN rn <= 10 THEN 0
WHEN rn <= 100 THEN 0.5
WHEN rn <= 200 THEN 0.25
WHEN rn <= 500 THEN 0.15
ELSE 0.05
END
)
FROM generate_series(1,t.user_limit) rn
)
FROM mytab t;