What year was there no competition in Women's singles?
CREATE TABLE "past_winners" (
  "year" text,
  "men_s_singles" text,
  "women_s_singles" text,
  "men_s_doubles" text,
  "women_s_doubles" text,
  "mixed_doubles" text
);
SELECT "year" FROM "past_winners" WHERE "women_s_singles"='no competition';