Which Rank has a Capacity smaller than 12,000, and a Country of united states?
CREATE TABLE "davis_cup_and_federation_cup_venues" (
  "rank" real,
  "stadium" text,
  "capacity" real,
  "city" text,
  "country" text
);
SELECT SUM("rank") FROM "davis_cup_and_federation_cup_venues" WHERE "capacity"<'12,000' AND "country"='united states';