Check out example codes for "how to get the date diff on once field in sql server". It will help you in understanding the concepts better.

Code Example 1

SELECT t1.OrderNo,DATEDIFF(day,t1.LoadedStartDate,t2.LoadedStartDate)
FROM UnnamedTableFromQuestion t1
     UnnamedTableFromQuestion t2
         t1.OrderNo = t2.OrderNo
WHERE t1.OpNo = 1 and
      t2.OpNo = 4

select cur.unique_id_field, cur.seq_no, cur.date_created ,

  datediff(second, prv.date_created, cur.date_created) as diff_in_seconds

from yourtable as cur

  join yourtable as prv

    on cur.seq_no = prv.seq_no + 1;

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