What year did dave f. mctaggart win the men's singles and robert b. williams ethel marshall win the mixed doubles?
CREATE TABLE "canadian_national_championships_and_cana" (
  "year" real,
  "men_s_singles" text,
  "women_s_singles" text,
  "men_s_doubles" text,
  "women_s_doubles" text,
  "mixed_doubles" text
);
SELECT MAX("year") FROM "canadian_national_championships_and_cana" WHERE "men_s_singles"='dave f. mctaggart' AND "mixed_doubles"='robert b. williams ethel marshall';