UPDATE Item SET AttachementID = NULL
    FROM Item i
    JOIN BidItem ebi ON i.BidItemID = ebi.BidItemID
    JOIN Group eg ON ebi.GroupID = eg.GroupID
    WHERE eg.EstimateID = @JobID and i.AttachementID NOT IN (
        SELECT i.ItemID FROM Item i
            JOIN BidItem ebi ON i.BidItemID = ebi.BidItemID
            JOIN Group eg ON ebi.GroupID = eg.GroupID
            WHERE eg.EstimateID = @JobID
    )

UPDATE Item SET AttachementID = NULL
    FROM Item i
    JOIN BidItem ebi ON i.BidItemID = ebi.BidItemID
    JOIN Group eg ON ebi.GroupID = eg.GroupID
    WHERE eg.EstimateID = @JobID

    and not exists
    (
            SELECT null FROM Item i
            JOIN BidItem ebi ON i.BidItemID = ebi.BidItemID
            JOIN Group eg ON ebi.GroupID = eg.GroupID
            WHERE eg.EstimateID = @JobID
            /* now "relate" to the outside query */
            and  i.ItemID = i.AttachementID
    )