Функция COALESCE в операторе OVER не работает

Может кто-нибудь сказать мне, почему COALESCE работает над первым SELECT здесь, а не над двумя другими? Я все еще получаю значения NULL для вторых двух утверждений.

(SELECT COALESCE(DEFax, NULL, '') FROM Debtor d WHERE d.DEIsPrimary = 1 AND d.CApKey = c.CApKey) AS FaxNumberOne,
        (SELECT COALESCE(DEFax, NULL, '') FROM (SELECT ROW_NUMBER() OVER (ORDER BY DEpKey ASC) 
            AS rownumber, DEFax FROM Debtor d WHERE d.CApKey = c.CApKey AND d.DEIsPrimary <> 1) 
            AS foo WHERE rownumber = 1) AS FaxNumberTwo,
        (SELECT COALESCE(DEFax, NULL, '') FROM (SELECT ROW_NUMBER() OVER (ORDER BY DEpKey ASC) 
            AS rownumber, DEFax FROM Debtor d WHERE d.CApKey = c.CApKey AND d.DEIsPrimary <> 1) 
            AS foo WHERE rownumber = 2) AS FaxNumberThree

Спасибо!