Check out example codes for "how to get the date diff of 2 dates in the same fieldin 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
       INNER JOIN
     UnnamedTableFromQuestion t2
       on
         t1.OrderNo = t2.OrderNo
WHERE t1.OpNo = 1 and
      t2.OpNo = 4
      
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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;

Code Example 2

WITH Partitioned AS(	SELECT *, ROW_NUMBER() OVER (PARTITION BY cu_id, date ORDER BY cu_id, date) AS RowNumber	FROM @Table) SELECT 	*,	CASE 		WHEN RowNumber > 1 THEN 0		ELSE COALESCE(DATEDIFF(DAY, (SELECT MAX(date) FROM @Table WHERE date < a.date AND cu_id = a.cu_id), a.date), 0)	END AS Days_betweenFROM Partitioned a

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