What year had a date of TBA where the Oakland raiders were the home team?
CREATE TABLE "game_history" (
  "year" real,
  "date" text,
  "television" text,
  "designated_visitors" text,
  "designated_home" text,
  "stadium" text,
  "city" text
);
SELECT AVG("year") FROM "game_history" WHERE "date"='tba' AND "designated_home"='oakland raiders';