On how many different dates was event 1 Suspension Bridge?
CREATE TABLE "episode_events" (
  "episode_number" real,
  "air_date" text,
  "event_1" text,
  "event_2" text,
  "event_3" text,
  "event_4" text
);
SELECT COUNT("air_date") FROM "episode_events" WHERE "event_1"='Suspension Bridge';