SQL LEFT JOIN with conditional CASE statements - sql

Question

Hopefully this is a quickie
SELECT *
FROM T
left JOIN J ON
CASE
WHEN condition1 THEN 1 --prefer this option even if CASE2 has a value
WHEN condition2 THEN 2
ELSE 0
END = 1 (edit: but if 1 does not satisfy, then join on 2)
Both cases return results, but I want THEN 1 to supersede THEN 2 and be the lookup priority
Can I have SQL do something like join on max(CASE)?
Basically I am trying to duplicate a nested INDEX/MATCH from Excel
edit: what i am hearing is that the Case should stop at the first returned TRUE, but it doesn't behave like that when i test
SELECT *
FROM T
left JOIN J ON
CASE
WHEN condition1 THEN 1 --prefer this option even if CASE2 has a value
WHEN condition2 THEN 1
ELSE 0
END = 1
it seems to prefer the 2nd THEN 1 sometimes, but not always... is there anything i am missing that would cause this behavior?

Answer 1

It doesn't matter which of the conditions causes the rows to match in a join. There are legitimate reasons to use a case expression in a join but I think you just want to or your conditions and then use the case expression to output a ranked reason for the match.
SELECT *, CASE WHEN <condition1> THEN 1 WHEN <condition2> THEN 2 END as match_code
FROM T LEFT OUTER JOIN J ON <condition1> or <condition2>
I don't know what to picture regarding the "nested INDEX/MATCH" from Excel. If I'm on the wrong track above then perhaps you're looking for a nested case expression?
Now if your conditions will have matches across different rows and you only want to keep one then...
WITH matches AS (
SELECT *, CASE WHEN <condition1> THEN 1 WHEN <condition2> THEN 2 END AS match_code
FROM T LEFT OUTER JOIN J ON <condition1> OR <condition2>
), ranked as (
SELECT *, MIN(match_code) OVER (PARTITION BY ???) AS keeper
FROM matches
)
SELECT ...
FROM ranked
WHERE match_code = keeper

Answer 2

Well, you can always have several conditions in your CASE Statements:
SELECT *
FROM T
left JOIN J ON
CASE
WHEN condition1 THEN 1 --prefer this option even if CASE2 has a value
WHEN condition2 And !condition1 THEN 2
ELSE 0
END = 1
--UPDATED--
If both of your conditions are required to match, but condition1 is optional then you can try this statement too:
SELECT *
FROM T
left JOIN J ON
CASE
WHEN condition1 And condition2 THEN 1 --Both conditions match
WHEN condition2 THEN 2 -- condition1 has no match
ELSE 0
END = 1

Answer 3

You can use With statement to this in 2 steps:
With first_join as
(SELECT *
FROM T
left JOIN J ON condition1)
select * from first_join
join J On case when Name_of_2nd_condition is null
then condition2
ELSE null end

Answer 4

You can use the CROSS APPLY /OUTER APPLY operator : https://www.mssqltips.com/sqlservertip/1958/sql-server-cross-apply-and-outer-apply/
SELECT *
FROM T
OUTER APPLY (SELECT TOP 1 *
FROM J
WHERE condition1 OR condition2
ORDER BY order) J