Consider Zipcode_list table with column Zipcode
Answer: Point to be noted is that the Maximum zipcode is not smaller than any Zipcode in the list.
We can use self join to find the list of Zipcodes that are smaller than at least one other Zipcode. Once we get that list, we just use NOT IN to find the Zipcode from Zipcode_list that does not exist in this smaller list. That will be the maximum Zipcode with no Zipcode bigger than it.
Query will be as follows:
SELECT DISTINCT Zipcode
WHERE Zipcode NOT IN (
FROM Zipcode_list AS Larger_list
JOIN Zipcode_list AS Smaller_list
ON Smaller_list.Zipcode < Largerlist.Zipcode