When did the ship that was commissioned July 1948 with a bow number of ps-31 launch?
CREATE TABLE "ships_in_class" (
  "bow_number" text,
  "ship_name" text,
  "launched" text,
  "commissioned" text,
  "service" text
);
SELECT "launched" FROM "ships_in_class" WHERE "commissioned"='july 1948' AND "bow_number"='ps-31';