LISTING C: Grouping Web Site Hits by Session

SELECT a.site_url,
a.caller_ip,
  a.hit_datetime AS start_session,
  MIN(b.hit_datetime) AS end_session
  FROM (SELECT DISTINCT site_url, caller_ip, hit_datetime
    FROM web_hits m
    WHERE NOT EXISTS (SELECT * FROM web_hits m2
      WHERE m2.site_url = m.site_url
      AND m2.caller_ip = m.caller_ip
      AND m2.hit_datetime < m.hit_datetime
      AND DATEADD(MINUTE, 30, m2.hit_datetime)>=m.hit_datetime )
    ) a JOIN (
  SELECT DISTINCT site_url, caller_ip, hit_datetime
    FROM web_hits m
    WHERE NOT EXISTS (SELECT * FROM web_hits m2
      WHERE m2.site_url = m.site_url
      AND m2.caller_ip = m.caller_ip
      AND DATEADD(MINUTE, 30, m2.hit_datetime) >
      DATEADD(MINUTE, 30, m.hit_datetime)
      AND m2.hit_datetime<= DATEADD(MINUTE, 30, m.hit_datetime))
    ) b ON
  b.site_url = a.site_url
  AND b.caller_ip = a.caller_ip
  AND DATEADD(MINUTE, 30, b.hit_datetime) >= a.hit_datetime
GROUP BY a.site_url, a.caller_ip, a.hit_datetime