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

Question: Give the earliest year that featured Pi Hongyan on women's singles.
SQL: SELECT MIN("year") FROM "previous_winners" WHERE "women_s_singles"='pi hongyan';

Question: What's the first year that Guo Xin featured in women's singles?
SQL: SELECT MIN("year") FROM "previous_winners" WHERE "women_s_singles"='guo xin';