Which song was sung by Artist Luther Vandross in the Week Top 20?
CREATE TABLE "list_of_performances_on" (
  "week" text,
  "theme" text,
  "song_sung" text,
  "artist" text,
  "status" text
);
SELECT "song_sung" FROM "list_of_performances_on" WHERE "artist"='luther vandross' AND "week"='top 20';