Using CASE in WHERE clause - sql

Question

I have 2 tables that needs to be joined based on couple of parameters. One of the parameters is year. One table contains the current year but another year doesn't contain current year, so it must use the latest year and matched with other parameters.
Example
Product
-------------------------------------------------------------------------------
| product_id | category_id | sub_category_id | product_year | amount |
-------------------------------------------------------------------------------
| 504 | I | U | 2020 | 400 |
| 510 | I | U | 2019 | 100 |
| 528 | I | U | 2019 | 150 |
| 540 | I | U | 2018 | 1000 |
Discount
-----------------------------------------------------------------------------
| discount_year | category_id | sub_category_id | discount |
-----------------------------------------------------------------------------
| 2018 | I | U | 0.15 |
| 2017 | I | U | 0.35 |
| 2016 | I | U | 0.50 |
Output
-----------------------------------------------------------------------------
| product_id | category_id | sub_category_id | product_year | discount_year |
-----------------------------------------------------------------------------
| 504 | I | U | 2020 | 2018 |
| 510 | I | U | 2019 | 2018 |
| 528 | I | U | 2019 | 2018 |
| 540 | I | U | 2018 | 2017 |
The discount is always gotten from one year behind but if those rates aren't available, then it would keep going back a year until available.
I have tried the following:
SELECT
product_year, a.product_id, a.category_id, a.sub_category_id,
discount_year, amount, discount
FROM
Product a
INNER JOIN
Discount b ON a.category_id = b.category_id
AND a.sub_category_id = b.sub_category_id
AND product_ year = CASE
WHEN discount_year + 1 = product_year
THEN discount_year + 1
WHEN discount_year + 2 = product_year
THEN discount_year + 2
WHEN discount_year + 3 = product_year
THEN discount_year + 3
END
WHERE
product = 540
This return the following output:
--------------------------------------------------------------------------------------------------
| product_year | product_id | category_id | sub_category_id | discount_year | amount | discount |
--------------------------------------------------------------------------------------------------
| 2016 | 540 | I | U | 2017 | 1000 | 0.50 |
| 2017 | 540 | I | U | 2017 | 1000 | 0.35 |
Any help will be appreciated.

Answer 1

You can use OUTER APPLY and a subquery. In the subquery select the row with the maximum discount_year, that is less the product_year using TOP and ORDER BY.
SELECT p.product_year,
p.product_id,
p.category_id,
p.sub_category_id,
d.discount_year,
p.amount,
d.discount
FROM product p
OUTER APPLY (SELECT TOP 1
*
FROM discount d
WHERE d.category_id = p.category_id
AND d.sub_category_id = p.sub_category_id
AND d.discount_year < p.product_year
ORDER BY d.discount_year DESC) d;

Answer 2

instead of a CASE expression you can use a sub-query to select the TOP 1 related
discount_year that is less than your product_year, ORDER BY discount_year ASC.

Answer 3

Create a product to discount mapping using a CTE first. This contains the discount year pulled from discount table for every product year in the product table and corresponding product_id. Following this, you can easily join with relevant tables to get results and eliminate any nulls as needed
Simplified query.
;WITH disc_prod_mapper
AS
(
SELECT product_id, product_year,(SELECT MAX(discount_year) FROM #Discount b WHERE discount_year < product_year AND a.category_id = b.category_id AND a.sub_category_id = b.sub_category_id ) AS discount_year
FROM Product a
)
SELECT a.product_year, c.discount_year, a.amount, c.discount
FROM Product a
LEFT JOIN disc_prod_mapper b ON a.product_id = b.product_id
LEFT JOIN Discount c ON b.discount_year = c.discount_year