What is the difference between the two following Selects. It seems that they should return the same results, but the "NOT IN" produces 0 records and the EXISTS clause produces multiple rows. I thought that they should return the same result.

Using "NOT EXISTS":
SELECT DISTINCT u1.[vcRequestor]
FROM dbPDRProductionCopy..tbl_PDR u1
WHERE NOT EXISTS
(SELECT u2.[TolasKey]
FROM Core..User_tbl u2
WHERE u1.[vcRequestor] = u2.[TolasKey])

Using "NOT IN":

SELECT DISTINCT u1.[vcRequestor]
FROM dbPDRProductionCopy..tbl_PDR u1
WHERE u1.[vcRequestor] NOT IN
(SELECT u2.[TolasKey]
FROM Core..User_tbl u2)