SQL Sever: in...case...in WHERE clause - tsql

Question

I need to code up a query for something like this:
Select [something]
Where
condition in
case
when (if another_condition = A and 3rd Condition = B) then (C,D)
when (if another_condition = N and 3rd Condition = E) then (F,G)
else (J,K)
end
essentially, what I want is if A and B are met, condition could be set to either C or D, if N or E are met, then condition could be set to F or G, else condition set to J or K.
However, when I run this, I kept getting
Incorrect syntax near the keyword 'Case'.
Please help! Thanks!

Answer 1

Maybe this:
Where (Another_Condition = 'A' And Third_Condition = 'B' And Condition in ('C','D'))
Or
(Another_Condition = 'N' and Third_Condition = 'E' And Condition in ('F','G'))
Or
Condition In ('J','K')
Be very careful about mixing and's and or's in a where clause. Parenthesis are important.

Answer 2

How about this - the UNION subquery will give you the full result set within the subquery. Then you can say 'WHERE condition IN ' (subquery). Like this:
SELECT [something]
WHERE
condition IN
(SELECT CASE WHEN (another_condition = A AND 3rd Condition = B) THEN C
WHEN (another_condition = N AND 3rd Condition = E) THEN F
ELSE J
END AS Value
UNION
SELECT CASE WHEN (another_condition = A AND 3rd Condition = B) THEN D
WHEN (another_condition = N AND 3rd Condition = E) THEN G
ELSE K
END AS Value
)

Answer 3

I'd probably go with G Mastro's approach of expanding the query as a Boolean expression. While the nested query approach will work, the intent of the code is less obvious IMO.
Having said that, if there are a lot of cases in your CASE statement, you may want to consider reshaping your data, because no matter how you write the query, it boils down to a big Boolean expression.