Group position of 1st, and a Result F–A of 2–0 has what average attendance?
CREATE TABLE "group_stage" (
  "date" text,
  "opponents" text,
  "result_f_a" text,
  "attendance" real,
  "group_position" text
);
SELECT AVG("attendance") FROM "group_stage" WHERE "group_position"='1st' AND "result_f_a"='2–0';