Sql Query Pervious Row Optimisation

Question

Here is my table structure

MyTable
-----------

ObjectID int (Identity),           -- Primary Key
FileName varchar(10),
CreatedDate datetime
...........
....         


        

Answer 1

SELECT t1.FileName, t1.CreatedDate, t2.CreatedDate as PrevCreatedDate
FROM 
   (SELECT FileName, CreateDate,
          ROW_NUMBER() OVER(PARTITION BY FileName ORDER BY CreatedDate) AS OrderNo
   FROM MyTable) t1
LEFT JOIN
   (SELECT FileName, CreateDate,
     ROW_NUMBER() OVER(PARTITION BY FileName ORDER BY CreatedDate) AS OrderNo
     FROM MyTable) t2
ON (t1.FileName = t2.FileName AND t1.OrderNo = t2.OrderNo - 1)

Or may be better use 'WITH', because queries is identical:

WITH t(ObjectID, FileName, CreatedDate, OrderNo) AS
   (SELECT ObjectID, FileName, CreatedDate,
          ROW_NUMBER() OVER(PARTITION BY FileName ORDER BY CreatedDate) AS OrderNo
   FROM MyTable) 
SELECT t1.ObjectID, t1.FileName, t1.CreatedDate, t2.CreatedDate AS PrevCreatedDate,
        DATEDIFF("SS", '1900-01-01 00:00:00', 
           COALESCE((t1.CreatedDate - t2.CreatedDate),0)) AS secondsTaken
FROM t t1 LEFT JOIN t t2 
ON (t1.FileName = t2.FileName AND t1.OrderNo = t2.OrderNo + 1)