What is the mean round number for center position when the pick number is less than 23?
CREATE TABLE "atlanta_falcons_draft_history" (
  "round" real,
  "pick_num" real,
  "overall" real,
  "name" text,
  "position" text,
  "college" text
);
SELECT AVG("round") FROM "atlanta_falcons_draft_history" WHERE "position"='center' AND "pick_num"<23;