What is the average Year, when Country is "U.S.", and when Location is "Edmond , OK"?
CREATE TABLE "workplace_killings" (
  "perpetrator" text,
  "date" text,
  "year" real,
  "location" text,
  "country" text,
  "killed" real,
  "injured" text
);
SELECT AVG("year") FROM "workplace_killings" WHERE "country"='u.s.' AND "location"='edmond , ok';