SELECT DISTINCT a,b,c FROM t
es aproximadamente equivalente a:
SELECT a,b,c FROM t GROUP BY a,b,c
Es una buena idea acostumbrarse a la sintaxis GROUP BY, ya que es más potente.
Para su consulta, lo haría así:
UPDATE sales
SET status='ACTIVE'
WHERE id IN
(
SELECT id
FROM sales S
INNER JOIN
(
SELECT saleprice, saledate
FROM sales
GROUP BY saleprice, saledate
HAVING COUNT(*) = 1
) T
ON S.saleprice=T.saleprice AND s.saledate=T.saledate
)