What is the total launch failures when there are 4 launches, and the not usable is greater than 0?
CREATE TABLE "satellites_by_block" (
  "block" text,
  "launched" real,
  "operational" real,
  "not_usable" real,
  "retired" real,
  "launch_failures" real
);
SELECT SUM("launch_failures") FROM "satellites_by_block" WHERE "launched"=4 AND "not_usable">0;