做個查詢，字段1值除3，如果=1，就用字段2值-20，如果=2，字段2 值-30，將差值大于0的數據選出來?

SELECT col1, col2,?
CASE?
???WHEN col1 / 3 = 1 THEN col2 - 20?
???WHEN col1 / 3 = 2 THEN col2 - 30
END AS difference
FROM price
WHERE (
CASE?
???WHEN col1 / 3 = 1 THEN col2 - 20?
???WHEN col1 / 3 = 2 THEN col2 - 30
END > 0)