When 4.5 is the rating (18-49) what is the air date?
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 "first_air_date" FROM "u_s_nielsen_ratings" WHERE "rating_18_49"='4.5';