How to update one table from another table having more than 1 row with matching conditions? - in postgreSQL - postgresql

Question

I have table A with product_id,cost,year,quarter,... etc columns.
I have another table B with product_id,base_cost,current_year,p_year,p_quarter,p_order columns.
I want to write an update query to update A from B. My conditions are -
WHERE A.product_id=B.product_id
and A.year=B.current_year
and A.year=B.p_year and A.quarter>B.p_quarter
and A.cost=0;
But the problem is, with these conditions if i have more than one rows in B then i only want to update from the row of B which has the minimum of all quarters.
Example 1-
If A has one row as:
product_id cost year quarter
102 0 2019 1
102 0 2019 2
102 0 2019 3
102 0 2019 4
And B has two rows corresponding to the where clause:
product_id cost current_year p_year quarter
102 3.5 2019 2019 3
102 1.8 2019 2019 1
102 0.5 2019 2019 2
Then updated A should be:
product_id cost year quarter
102 0 2019 1
102 1.8 2019 2
102 1.8 2019 3
102 1.8 2019 4

Answer 1

This is a greatest-n-per-group problem which you need to apply to the table b in a sub-select:
UPDATE A
SET cost = B.base_cost
FROM (
select distinct on (product_id, current_year) product_id, current_year, cost
from b
order by product_id, current_year, quarter
) b
WHERE A.product_id = B.product_id
AND A.year = B.current_year
AND A.cost = 0