What were the names of the mixed doubles pair for the year that Ralph Nichols played in the men's singles match?
CREATE TABLE "winners" (
  "year" text,
  "men_s_singles" text,
  "men_s_doubles" text,
  "women_s_doubles" text,
  "mixed_doubles" text
);
SELECT "mixed_doubles" FROM "winners" WHERE "men_s_singles"='ralph nichols';