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: What is the average year with alfredo salazar fina salazar in mixed doubles?
SQL: SELECT AVG("year") FROM "previous_winners" WHERE "mixed_doubles"='alfredo salazar fina salazar';

Question: Which women's doubles happened after 1987 and a women's single of astrid van der knaap?
SQL: SELECT "women_s_doubles" FROM "previous_winners" WHERE "year">1987 AND "women_s_singles"='astrid van der knaap';