How to group ranged values using SQL Server

Question

I have a table of values like this

978412, 400
978813, 20
978834, 50
981001, 20

As you can see the second number when added to the first is

Answer 1

From the article that Josh posted, here's my take (tested and working):

SELECT
    MAX(t1.gapID) as gapID,
    t2.gapID-MAX(t1.gapID)+t2.gapSize as gapSize
    -- max(t1) is the specific lower bound of t2 because of the group by.
FROM
  ( -- t1 is the lower boundary of an island.
    SELECT gapID
    FROM gaps tbl1 
    WHERE
      NOT EXISTS(
        SELECT *
        FROM gaps tbl2 
        WHERE tbl1.gapID = tbl2.gapID + tbl2.gapSize + 1
      )
  ) t1
  INNER JOIN ( -- t2 is the upper boundary of an island.
    SELECT gapID, gapSize
    FROM gaps tbl1 
    WHERE
      NOT EXISTS(
        SELECT * FROM gaps tbl2 
        WHERE tbl2.gapID = tbl1.gapID + tbl1.gapSize + 1
      )
  ) t2 ON t1.gapID <= t2.gapID -- For all t1, we get all bigger t2 and opposite.
GROUP BY t2.gapID, t2.gapSize

Answer 2

Check out this MSDN Article. It gives you a solution to your problem, if it will work for you depends on the ammount of data you have and your performance requirements for the query.

Edit:

Well using the example in the query, and going with his last solution the second way to get islands (first way resulted in an error on SQL 2005).

SELECT MIN(start) AS  startGroup, endGroup, (endgroup-min(start) +1) as NumNodes
FROM (SELECT g1.gapID AS start,
(SELECT min(g2.gapID) FROM #gaps g2 
WHERE g2.gapID >= g1.gapID and NOT EXISTS
(SELECT * FROM #gaps g3 
WHERE g3.gapID - g2.gapID = 1)) as endGroup
FROM #gaps g1) T1 GROUP BY endGroup

The thing I added is (endgroup-min(start) +1) as NumNodes. This will give you the counts.