What is the average height for hewitt class, with prom less than 86, and a Peak of gragareth?
CREATE TABLE "southern_fells" (
  "peak" text,
  "height_m" real,
  "prom_m" real,
  "class" text,
  "parent" text
);
SELECT AVG("height_m") FROM "southern_fells" WHERE "class"='hewitt' AND "prom_m"<86 AND "peak"='gragareth';