CREATE TABLE "past_winners" (
  "year" real,
  "men_s_singles" text,
  "women_s_singles" text,
  "men_s_doubles" text,
  "women_s_doubles" text,
  "mixed_doubles" text
);

Question: What is the average year with Nathan Robertson Gail Emms in mixed doubles?
SQL: SELECT AVG("year") FROM "past_winners" WHERE "mixed_doubles"='nathan robertson gail emms';