What is the average number of not usable satellited when there are 0 retired, and the launch failures are less than 1?
CREATE TABLE "satellites_by_block" (
  "block" text,
  "launched" real,
  "operational" real,
  "not_usable" real,
  "retired" real,
  "launch_failures" real
);
SELECT AVG("not_usable") FROM "satellites_by_block" WHERE "retired"=0 AND "launch_failures"<1;