T-SQL use different WHERE clause based on value of input parameter - sql-server

Question

I would like to change the WHERE clause of a query based upon the value of an input parameter. I'm guessing this is possible, but it seems that I'm approaching it in the wrong way?
A simplified version on my SP query:
CREATE PROCEDURE [dbo].[GetMailboxMessagesByStatus]
#UserId UNIQUEIDENTIFIER,
#MessageStatus INT
AS
BEGIN
SELECT *
FROM MailboxMessages m
WHERE
CASE #MessageStatus
WHEN 4 THEN m.SenderId = #UserId --Sent
ELSE m.RecipientId = #UserId --Inbox
END
END
GO

Answer 1

No need for case or iif constructs:
WHERE #MessageStatus = 4 AND m.SenderId = #UserId
OR #MessageStatus <> 4 AND m.RecipientId = #UserId
EDIT:
Be aware on big tables using this construct when the table being queried is quite large. Using 2 seperate queries using a IF statement like Chester Lim suggested might be the better solution. Preventing parameter sniffing might also be a good idea

Answer 2

Use an IF statement.
CREATE PROCEDURE [dbo].[GetMailboxMessagesByStatus]
#UserId UNIQUEIDENTIFIER ,
#MessageStatus INT
AS
BEGIN
IF ( #MessageStatus = 4 )
BEGIN
SELECT *
FROM MailboxMessages
WHERE m.SenderId = #UserId; --Sent
END;
ELSE
BEGIN
SELECT *
FROM MailboxMessages m
WHERE m.RecipientId = #UserId; --Inbox
END;
END;
GO
EDIT - a much better way provided by LukStorms (since i did not know IIF until i saw his answer)
CREATE PROCEDURE [dbo].[GetMailboxMessagesByStatus]
#UserId UNIQUEIDENTIFIER ,
#MessageStatus INT
AS
BEGIN
SELECT *
FROM MailboxMessages m
WHERE IIF (#MessageStatus = 4, m.SenderId, m.RecipientId) = #UserId; --Sent
END
GO

Answer 3

You could change that WHERE clause to
WHERE (CASE WHEN #MessageStatus = 4 THEN m.SenderId ELSE m.RecipientId END) = #UserId
Because what you put after the THEN in a CASE should just be a value, not a comparison.
Or use IIF instead of a CASE:
WHERE IIF(#MessageStatus = 4,m.SenderId,m.RecipientId) = #UserId
But the SQL will run more efficient if you use an IF ... ELSE ... and run a different Query based on the #MessageStatus.
Was writing an example for that, but Chester Lim already beat me to it. ;)
(so no need to repeat that approach in this answer)