How to use IF within AND in T-SQL - sql

Question

I am trying to create a stored procedure which will use one of the optional parameters provided.
In the code below they are #nStudentId and #nStudentIds set to NULL initially. Only one of them will be sent when the proc is called. When #nStudentIds are sent they will come-in as comma separated values.
CREATE PROCEDURE [dbo].[usp_GetStudentReferrals]
(
#ProfessorId BIGINT,
#nStudentId BIGINT = NULL,
#nStudentIds NVARCHAR(999) = NULL
)
AS
BEGIN
SELECT DISTINCT SR.StudentReferralId
FROM StudentReferral SR WITH(NOLOCK)
INNER JOIN PotentialCandidate PC WITH(NOLOCK) ON PC.PotentialCandidateId = SR.StudentId
WHERE SR.ProfessorId = #nProfessorId
AND -- this is where I am not able to figure out the logic to use either #nStudentId or #nStudentIds, whichever is passed in.
END
So when #nStudentId is sent it should be this in the AND
SR.StudentId = #nStudentId
When #nStudentIds is available I can use 'IN' like so:
SR.StudentId IN (SELECT value FROM STRING_SPLIT #nStudentIds, ','))
The limitation of my knowledge of SQL shows in this IF, which obviously does not work:
AND (if(#nStudentId <> 0 AND #nStudentId <> -1 AND #nStudentId IS NULL)
SR.StudentId = #nStudentId;
else if(#nStudentIds IS NOT NULL)
SR.StudentId IN (SELECT value FROM STRING_SPLIT#nStudentIds,','))
)
Any suggestions are appreciated.
Thanks in advance.
Regards.

Answer 1

There's literally no need for 2 parameters here. Ideally, what you should be using is a table type parameter, as that maintains the strong typing. Then you can just JOIN to said table type parameter:
CREATE TYPE dbo.IDs AS table (ID bigint);
GO
CREATE PROC dbo.usp_GetStudentReferrals #ProfessorId bigint, #StudentIDs dbo.IDs READONLY AS
BEGIN
SELECT DISTINCT SR.StudentReferralId
FROM dbo.StudentReferral SR --WITH(NOLOCK) --Why are you using NOLOCK? You do know what it does, right?
INNER JOIN dbo.PotentialCandidate PC /*WITH(NOLOCK)*/ ON PC.PotentialCandidateId = SR.StudentId --Why are you using NOLOCK? You do know what it does, right?
INNER JOIN #StudentIDs S ON SR.StudentID = S.ID
WHERE SR.ProfessorId = #ProfessorId; --I assumed this should be #ProfessorId not #nProfessorId
END;
GO
Then you would call the procedure with something like:
DECLARE #ProfessorId bigint, #Students dbo.IDs;
SET #ProfessorId = 123456789;
INSERT INTO #Students (ID)
VALUES(987654321),(5643321987342);
EXEC dbo.usp_GetStudentReferrals #ProfessorId, #Students;

Answer 2

why don't you put IF before SELECT
BEGIN
if(#nStudentId <> 0 AND #nStudentId <> -1 AND #nStudentId IS NULL)
SELECT DISTINCT .....
else
SELECT DISTINCT .....
you can using split string from
link
and change this part
AND (if(#nStudentId <> 0 AND #nStudentId <> -1 AND #nStudentId IS NULL)
SR.StudentId = #nStudentId;
else if(#nStudentIds IS NOT NULL)
SR.StudentId IN (SELECT name FROM dbo.splitstring(#nStudentIds))
)