What is the smallest height (ft) of a building in Frankfurt with a height (m) of 257 and less than 55 floors?
CREATE TABLE "timeline_of_tallest_buildings" (
  "name" text,
  "city" text,
  "height_m" real,
  "height_ft" real,
  "floors" real,
  "years_as_tallest" text
);
SELECT MIN("height_ft") FROM "timeline_of_tallest_buildings" WHERE "city"='frankfurt' AND "height_m"=257 AND "floors"<55;