What is the total number of medals won by nations that had 7 bronze medals and more than 18 gold medals?
CREATE TABLE "performance_by_nation" (
  "rank" text,
  "gold" real,
  "silver" real,
  "bronze" real,
  "total" real
);
SELECT SUM("total") FROM "performance_by_nation" WHERE "bronze"=7 AND "gold">18;