You will not get the results you want if the values are NULL.
When I compare tables I use a CHECKSUM.
SELECT
*
FROM
(SELECT
[compareHR].[dbo].[TableOLD].[Pos_no],
CHECKSUM(*) AS XSum
FROM
[compareHR].[dbo].[TableOLD]) AS Old
FULL JOIN
(SELECT
[compareHR].[dbo].[TableNEW].[Pos_no],
CHECKSUM(*) AS XSum
FROM
[compareHR].[dbo].[TableNEW]) AS New
WHERE
Old.Pos_no IS NULL -- Row doesn't exist in old table
OR New.Pos_no IS NULL -- Row doesn't exist in new table
OR Old.XSum <> New.XSum; -- Row is different
