Creating a Stored Procedure with if statement on Inner Join - sql-server

Question

I am trying to create a stored procedure which would execute an INNER JOIN code block based on a parameter value. However, I keep getting "Incorrect syntax near '#reSourceID'."
if (#VendorID = 11)
#reSourceID = 't.reSourceID'
if (#VendorID = 5)
#reSourceID = 't.SourceID'
SELECT t.ID, fsg.SigCap, fsg.VendorId
FROM FormCap fsg
INNER JOIN FlightTrip t
ON fsg.SourceId = #reSourceID
AND fsg.VendorId = #VendorID
INNER JOIN ContractProvider cpu
ON t.Id = cpu.VendorId
WHERE (t.ID = #FinTransID)
AND (cpu.userID = #UserID)
Any ideas what would be causing the error?

Answer 1

Your syntax error is because you haven't really joined on FlightTrip t. You can't do what you are trying to do in compiled SQL. You would have to create a varchar variable full of your statement and then use EXEC SP_EXECUTESQL(#Statement) to run it.
declare #statement nvarchar(4000)
select #statement = '
SELECT t.ID,
fsg.SigCap,
fsg.VendorId
FROM FormCap fsg
INNER JOIN FlightTrip t
ON fsg.SourceId = '+convert(nvarchar(100),#reSourceID)+'
AND fsg.VendorId = '+convert(nvarchar(100),#VendorID)+'
INNER JOIN ContractProvider cpu
ON t.Id = cpu.VendorId
WHERE t.ID = '+convert(nvarchar(100),#FinTransID)+'
AND cpu.userID = '''+convert(nvarchar(100),#UserID)+''''
exec sp_executesql #statement

Answer 2

SELECT t.ID, fsg.SigCap, fsg.VendorId
FROM FormCap fsg
INNER JOIN FlightTrip t
ON fsg.SourceId = CASE #VendorID
WHEN 11 THEN t.reSourceID
WHEN 5 THEN t.SourceID
END
AND fsg.VendorId = #VendorID
INNER JOIN ContractProvider cpu
ON t.Id = cpu.VendorId
WHERE (t.ID = #FinTransID)
AND (cpu.userID = #UserID)

Answer 3

As Bill answered, the optimim solution is probably going to involve dynamic SQL although I'd recommend going a little farther both for performance and security.
If you parameterize your call to sp_executeSQL you'll both avoid issues with escaping quotes and potential SQL injection problems but it will allow SQL to reuse the plan since the query text won't change.
Here's what that would look like:
if (#VendorID = 11)
#reSourceID = 't.reSourceID'
if (#VendorID = 5)
#reSourceID = 't.SourceID'
DECLARE #sql NVARCHAR(MAX)
DECLARE #params NVARCHAR(500)
SET #sql ='SELECT t.ID,
fsg.SigCap,
fsg.VendorId
FROM FormCap fsg
INNER JOIN FlightTrip t
ON fsg.SourceId = '+CONVERT(NVARCHAR(MAX),#reSourceID)+'
AND fsg.VendorId = #VendorID
INNER JOIN ContractProvider cpu
ON t.Id = cpu.VendorId
WHERE t.ID = #FinTransID
AND cpu.userID = #userID'
SET #params = N'#VendorID INT, #FinTransID INT, #userID UNIQUEIDENTIFIER'
EXECUTE sp_executesql #sql, #params, #venderID = #venderID, #finTransID=#finTransID, #userID = #userID
Above is the gist of the solution, I don't fully know your types and inputs. Also, without schema, I can't test my code, but it should get your going.
For more information on sp_executesql, msnd is a great resource.