Perform an optional join on NULL parameter

optional.sql

 DECLARE @Today datetime = GETDATE()

 DECLARE @CampusId int
 SELECT @CampusId = 2
 DECLARE @CampusGuid uniqueidentifier
 SELECT @CampusGuid = '6246DDEF-8AC9-4C25-B011-430CEE590F0F'

SELECT COUNT(DISTINCT PersonAliasId)
FROM Attendance A
INNER JOIN (
	-- Return a value (or return null)
	SELECT (
		SELECT ID
		FROM Campus
		WHERE GUID = @CampusGuid
	) AS ID
) CampusFilter
ON A.CampusId = ISNULL(@CampusId, A.CampusId)
WHERE DidAttend = 1
	-- Filter by Date Range
	AND StartDateTime > DATEADD(month, -6, @Today)
	AND StartDateTime < @Today