What is the average Tier when the postseason shows -, and the season is 2012–13?
CREATE TABLE "season_by_season" (
  "season" text,
  "tier" real,
  "league" text,
  "postseason" text,
  "european_competitions" text
);
SELECT AVG("tier") FROM "season_by_season" WHERE "postseason"='–' AND "season"='2012–13';