Mesclar linhas adjacentes no SQL?

Este artigo fornece algumas soluções possíveis para sua pergunta

http://www.sqlmag.com/blog/puzzled-by-t-sql-blog-15/tsql/solutions-to-packing-date-and-time-intervals-puzzle-136851

Este parece ser o mais direto:

WITH StartTimes AS
(
  SELECT DISTINCT username, starttime
  FROM dbo.Sessions AS S1
  WHERE NOT EXISTS
    (SELECT * FROM dbo.Sessions AS S2
     WHERE S2.username = S1.username
       AND S2.starttime < S1.starttime
       AND S2.endtime >= S1.starttime)
),
EndTimes AS
(
  SELECT DISTINCT username, endtime
  FROM dbo.Sessions AS S1
  WHERE NOT EXISTS
    (SELECT * FROM dbo.Sessions AS S2
     WHERE S2.username = S1.username
       AND S2.endtime > S1.endtime
       AND S2.starttime <= S1.endtime)
)
SELECT username, starttime,
  (SELECT MIN(endtime) FROM EndTimes AS E
   WHERE E.username = S.username
     AND endtime >= starttime) AS endtime
FROM StartTimes AS S;