Of all the schools that left before 2003, what is the average year joined?
CREATE TABLE "membership" (
  "school" text,
  "location" text,
  "mascot" text,
  "county" text,
  "year_joined" real,
  "previous_conference" text,
  "year_left" real,
  "conference_joined" text
);
SELECT AVG("year_joined") FROM "membership" WHERE "year_left"<2003;