What average week has shea stadium as the game site, and 1979-12-09 as the date?
CREATE TABLE "schedule" (
  "week" real,
  "date" text,
  "opponent" text,
  "result" text,
  "game_site" text,
  "attendance" real
);
SELECT AVG("week") FROM "schedule" WHERE "game_site"='shea stadium' AND "date"='1979-12-09';