What is the On-air ID with the frequency of 105.9, and purpose of commercial?
CREATE TABLE "defunct_callsigns" (
  "callsign" text,
  "area_served" text,
  "frequency" text,
  "band" text,
  "on_air_id" text,
  "purpose" text
);
SELECT "on_air_id" FROM "defunct_callsigns" WHERE "frequency"='105.9' AND "purpose"='commercial';