Which mixed doubles was featured in 2006?
CREATE TABLE "21st_century" (
  "year" real,
  "men_s_singles" text,
  "women_s_singles" text,
  "men_s_doubles" text,
  "women_s_doubles" text,
  "mixed_doubles" text
);
SELECT "mixed_doubles" FROM "21st_century" WHERE "year"=2006;