When 13 is the share (18-49) what is the rank (timeslot)?
CREATE TABLE "u_s_nielsen_ratings" (
  "episode" real,
  "first_air_date" text,
  "rating_18_49" text,
  "share_18_49" real,
  "viewers_millions" text,
  "rank_timeslot" text
);
SELECT "rank_timeslot" FROM "u_s_nielsen_ratings" WHERE "share_18_49"=13;