Which Men's 50 has a Senior Mixed of n/A, and a Men's Open of gold coast sharks def sydney mets, and a Men's 30 of sydney mets def sydney scorpions, and a Women's u20 of southern suns def brisbane city cobras?
CREATE TABLE "results_by_year" (
  "year" real,
  "men_s_open" text,
  "women_s_open" text,
  "mixed_open" text,
  "men_s_u20" text,
  "women_s_u20" text,
  "senior_mixed" text,
  "men_s_30" text,
  "women_s_30" text,
  "men_s_35" text,
  "women_s_35" text,
  "men_s_40" text,
  "women_s_40" text,
  "men_s_45" text,
  "men_s_50" text
);
SELECT "men_s_50" FROM "results_by_year" WHERE "senior_mixed"='n/a' AND "men_s_open"='gold coast sharks def sydney mets' AND "men_s_30"='sydney mets def sydney scorpions' AND "women_s_u20"='southern suns def brisbane city cobras';