How many years have an Opponent in the final of welby van horn?
CREATE TABLE "grand_slam" (
  "outcome" text,
  "year" real,
  "championship" text,
  "surface" text,
  "opponent_in_the_final" text,
  "score_in_the_final" text
);
SELECT SUM("year") FROM "grand_slam" WHERE "opponent_in_the_final"='welby van horn';