What was the attendance on 31 January 2009 when the opponent was Airdrie United?
CREATE TABLE "2008_09_partick_thistle_f_c_season" (
  "date" text,
  "opponent" text,
  "venue" text,
  "result" text,
  "attendance" real
);
SELECT AVG("attendance") FROM "2008_09_partick_thistle_f_c_season" WHERE "opponent"='airdrie united' AND "date"='31 january 2009';