SQL Server Case statement in WHERE Clause - sql-server

Question

I tried to google for CaseStatement in WHERE clause. But i didn't find similar to my scenario.
Below is my SQL Statement with CASE Statement in WHERE clause. If PartName = B, then i should apply (RecoveraleFlag = 1) condition along with other conditions. Else This condition should not apply but all other conditions should remain.
FROM Rec.Communications A
INNER JOIN REC.CommunicationTypes B ON A.CommunicationTypeKey = B.CommunicationTypeKey
INNER JOIN occ.Cases c ON a.CaseId = c.CaseId
INNER JOIN occ.Claims cl on a.CaseId = cl.CaseId
INNER JOIN ops.Concepts d ON c.ConceptKey = d.ConceptKey
INNER JOIN OPS.Regions f ON d.MODSRegionKey = f.MODSRegionKey
INNER JOIN COM.RepriceRequestOccurrences e ON a.CommunicationId = e.CommunicationId
INNER JOIN occ.Providers prv ON c.MODSProviderKey = prv.MODSProviderKey
WHERE
**(
CASE WHEN f.PartName = 'B' and e.RecoverableFlag = 1 then 1
ELSE 0
END
) = 1**
AND
b.CommunicationTypeCode = 'RREQ'
AND f.Region = #Region
AND a.CurrentFlag = 1
Here Case Statement in where clause is working fine if Partname = B. For Partname A, this will be 0=1 – always false. Because of this it is not returning any data.
Can anyone gives any alternatives.

Answer 1

As a case statement, you would write this as:
CASE WHEN f.PartName = 'B' and e.RecoverableFlag = 1 then 1
WHEN f.ParName = 'A' then 1
ELSE 0 END ) = 1
Is this the logic you want?
Many would think that the case statement is irrelevant here, and instead use:
WHERE ((f.PartName = 'B' and e.RecoverableFlag = 1) or (f.partName <> 'B')) . . .

Answer 2

WHERE
(
f.PartName <> 'B'
OR e.RecoverableFlag = 1
)
AND b.CommunicationTypeCode = 'RREQ'
AND f.Region = #Region
AND a.CurrentFlag = 1
You might find reading on De Morgan's Laws interesting.

Answer 3

I don't believe you need a case statement at all.. The following should work...
WHERE f.PartName = 'B'
And e.RecoverableFlag = 1
AND b.CommunicationTypeCode = 'RREQ'
AND f.Region = #Region
AND a.CurrentFlag = 1
But if you want to use a case statement in a where clause, try to put it on the "other" side of the comparison operator from the column name so the query can process it without doing a table scan...
as an e.g.,
Where f.Partname =
Case b.CommunicationTypeCode
When 'RREQ' Then 'B'
When 'NOTREQ' Then 'C'
When 'OPT' Then 'D'
Else 'F' End