SQL join best matcher - sql

Question

I have table A:
I need to join (SQL) this table onto table B, where I use ProductName as a join, but I want the following order of priorities:
Country being selected as a single row if it has a value (With Standard being null)
Using the combination of Country and Standard
Using Standard by itself (If Country being null).
I have tried looking around a lot. I hope the problem statement is clear.
Table A:
|ProductName|Country|Standard|Reportable|
|ProductA|Null|Value1|Y|
|ProductA|Value2|Value1|N|
|ProductA|Value2|Null|N|
The above is just a subset of the data, but basically the country and standard determine if the output is reportable. Product A could have 1 line or 3, depending on the data required.
Table B:
|ProductName|Year|
|ProductA|2006|
So the final join for the above should be:
|ProductName|Year|Country|Standard|Reportable|
|ProductA|2006|Value2|Null|N|

Answer 1

perhaps something like this: this is pseudo code at this time but hopefully gets the general concepts across.
Does assume A, B tables product are inner join related and not outer join.
the 1st CTE sets your priority. CTE (Common Table Expression)
the 2nd cte assigns a row number based on your priorities.
then the final query filters for the first row number encountered.
Obviously this is untested as we have no sample data or structure to test with at this time.
WITH CTE AS (
SELECT A.*
, B.*
, CASE WHEN A.Attribute is not null and B.attribute is null then 1
WHEN A.Attribute is not null and B.attribute is not null then 2
WHEN A.Attribute is null and B.Attribute is not null then 3 end as priority
FROM A
INNER JOIN B
on A.PRODUCT = B.PRODUCT),
CTE2 as (
SELECT CTE.*
, rowNumber() over (Order by Priority desc) RN
FROM CTE)
SELECT *
FROM CTE2
WHERE RN = 1

Answer 2

Something like this, maybe?
SELECT s.*, p.*
FROM source_table s
OUTER APPLY ( SELECT p.*
FROM product_table p
WHERE p.product_name = s.product_name
AND ( ( p.country = s.country AND s.standard IS NULL )
OR ( p.country = s.country AND p.standard = s.standard )
OR ( s.country IS NULL AND p.standard = s.standard )
ORDER BY CASE
WHEN ( p.country = s.country AND s.standard IS NULL ) THEN 1
WHEN ( p.country = s.country AND p.standard = s.standard ) THEN 2
WHEN ( s.country IS NULL AND p.standard = s.standard ) THEN 3
ELSE 99
FETCH FIRST 1 ROW ONLY )
OUTER APPLY (instead of CROSS APPLY) keeps your source_table result even if there is no product match. That may not be your desired outcome. If not, switch to CROSS APPLY.
There are probably ways to shorten the conditions and the sort order using NVL(). But I think this is the clearest way to start.
Also, if this is always a product match using one of those three conditions, you can shorten the WHERE clause in the OUTER APPLY.