CREATE TABLE weather (zip_code VARCHAR2, max_dew_point_f VARCHAR2)

Find all the zip codes in which the max dew point have never reached 70.
SELECT DISTINCT zip_code FROM weather EXCEPT SELECT DISTINCT zip_code FROM weather WHERE max_dew_point_f >= 70
SELECT DISTINCT "zip_code" FROM "weather" EXCEPT SELECT DISTINCT "zip_code" FROM "weather" WHERE "max_dew_point_f" >= 70