What is the average Decile with a state authority and a Name of mapiu school?
CREATE TABLE "waitomo_district" (
  "name" text,
  "years" text,
  "gender" text,
  "area" text,
  "authority" text,
  "decile" real,
  "roll" real
);
SELECT AVG("decile") FROM "waitomo_district" WHERE "authority"='state' AND "name"='mapiu school';