What was the surface where the outcome was winner on 12 march 1989?
CREATE TABLE "singles_37_19_18" (
  "outcome" text,
  "date" text,
  "tournament" text,
  "surface" text,
  "opponent_in_the_final" text,
  "score_in_the_final" text
);
SELECT "surface" FROM "singles_37_19_18" WHERE "outcome"='winner' AND "date"='12 march 1989';