What is the average year joined of the 277 county?
CREATE TABLE "membership" (
  "school" text,
  "location" text,
  "mascot" text,
  "county" real,
  "enrollment" text,
  "ihsaa_class" text,
  "year_joined" real,
  "previous_conference" text
);
SELECT AVG("year_joined") FROM "membership" WHERE "county"=277;