Replace nulls in table B with values in table A using SQL - sql

Question

I have a table like this:
price_families (Table A):
ID UPC
1 123
1 456
2 789
2 111
1 121
And a second table:
sales_volume (Table B):
UPC sales volume
123 13.99 2.99
456 null null
121 14.99 1.99
789 31.88 22.99
111 null null
121 null null
What I want is to replace the null values in Table B with the sales/volume values of a different UPC of the same product family (using Table A to determine price families, joining on UPC) and to order by sales desc, volume desc (for each price family).
What is the optimal way to do this? Can I use coalesce() here, or perhaps a case statement?
My output should be this:
output (Table C):
UPC sales volume
123 13.99 2.99
456 13.99 2.99
121 14.99 1.99
789 31.88 22.99
111 31.88 22.99
121 13.99 2.99

Answer 1

Hmmm . . . One method uses tuples for the assignment:
update b
set (sales, volume) = (select b2.sales, b2.volume
from b b2 join
a
on b2.upc = a.upc
where b2.sales is not null and b2.volume is not null
order by b2.sales desc, b2.volume desc
limit 1
)
where sales is null and volume is null;
EDIT:
If you just want the select query:
select b.upc,
coalesce(b.sales, b2.sales) as sales,
coalesce(b.volume, b2.volume) as volume
from b left join lateral
(select b2.sales, b2.volume
from b b2 join
a
on b2.upc = a.upc
where b2.sales is not null and b2.volume is not null
order by b2.sales desc, b2.volume desc
limit 1
) b2
on 1=1;