CREATE TABLE "previous_winners" (
  "year" real,
  "mens_singles" text,
  "womens_singles" text,
  "mens_doubles" text,
  "womens_doubles" text,
  "mixed_doubles" text
);

Question: How many years did Peter Mikkelsen win the Mens Singles?
SQL: SELECT COUNT("year") FROM "previous_winners" WHERE "mens_singles"='Peter Mikkelsen';

Question: Who won the Men's Doubles when Karina de Wit won the Women's Singles?
SQL: SELECT "mens_doubles" FROM "previous_winners" WHERE "womens_singles"='Karina de Wit';

Question: How many Mixed Doubles won when Oliver Pongratz won the Men's Singles?
SQL: SELECT COUNT("mixed_doubles") FROM "previous_winners" WHERE "mens_singles"='Oliver Pongratz';

Question: Who won the Men's Doubles when Guo Xin won the Women's Singles?
SQL: SELECT "mens_doubles" FROM "previous_winners" WHERE "womens_singles"='Guo Xin';