What is the average Year, when Outcome is "Winner"?
CREATE TABLE "pro_slam" (
  "outcome" text,
  "year" real,
  "pro_slam_championship" text,
  "surface" text,
  "opponent_in_the_final" text,
  "score_in_the_final" text
);
SELECT AVG("year") FROM "pro_slam" WHERE "outcome"='winner';