Re: SQL puzzle...

You haven't stated your rule for removing redundant rows. Assuming you want one arbitrary row of each (start,end) pair where Value is equal and assuming (start,end) is the primary key:

SELECT A.start, A.end, A.value
 FROM Graphe AS A
 LEFT JOIN Graphe AS B
  ON A.start = B.end

   AND A.end = B.start
   AND A.value = B.value
   AND A.start > B.start
 WHERE B.start IS NULL ;

This is SQL92 (apart from the use of the non-delimited reserved word "END" as a column name) but was tested on SQLServer 2000. Possibly you'll need to amend it depending on your Oracle version.

Hope this helps.

-- 
David Portas
SQL Server MVP
--