CREATE TABLE "atp_tour_venues" (
  "rank" text,
  "stadium" text,
  "capacity" real,
  "city" text,
  "country" text
);

Question: Which Capacity has a City of london, and a Stadium of queen's club?
SQL: SELECT AVG("capacity") FROM "atp_tour_venues" WHERE "city"='london' AND "stadium"='queen''s club';