Who played Mixed Doubles in 1991?
CREATE TABLE "world_championships_of_the_wbf" (
  "year" real,
  "mens_singles" text,
  "womens_singles" text,
  "mens_doubles" text,
  "womens_doubles" text,
  "mixed_doubles" text
);
SELECT "mixed_doubles" FROM "world_championships_of_the_wbf" WHERE "year"=1991;