Analytics data integrity is tough(pragmacoders.com)

   WITH page_timings AS (
     SELECT page_type
          , event_timestamp_utc
          , LEAD(event_timestamp_utc, 1)
              OVER(PARTITION BY session_id
                       ORDER BY event_timestamp_utc)
            AS next_pageview
       FROM events
      WHERE event_type = 'page_view'
    )
     SELECT page_type
          , MEDIAN(
              DATEDIFF(sec,
                       event_timestamp_utc,
                       next_pageview)
            ) AS median_sec_on_page
       FROM page_timings
      GROUP BY page_type
;