Date difference for same ID - sql-server

Question

I ve got a data set similar to
+----+------------+------------+------------+
| ID | Udate | last_code | Ddate |
+----+------------+------------+------------+
| 1 | 05/11/2018 | ACCEPTED | 13/10/2018 |
| 1 | 03/11/2018 | ATTEMPT | 13/10/2018 |
| 1 | 01/11/2018 | INFO | 13/10/2018 |
| 1 | 22/10/2018 | ARRIVED | 13/10/2018 |
| 1 | 15/10/2018 | SENT | 13/10/2018 |
+----+------------+------------+------------+
I m trying to get the date difference for each code on Udate, but for the first date I want to make datedifference between Udate and Ddate.
So I ve been trying:
DATEDIFF(DAY,LAG(Udate) OVER (PARTITION BY Shipment_Number ORDER BY Udate), Udate)
to get the difference between dates and it works so far, but I also need the first date difference between Udate and Ddate.
I was thinking about ISNULL()
Also, at the end I need an average of days between codes as well, usually they keep the same pattern. Sample output data:
+----+------------+------------+------------+------------+
| ID | Udate | last_code | Ddate | Difference |
+----+------------+------------+------------+------------+
| 1 | 05/11/2018 | ACCEPTED | 13/10/2018 | 2 |
| 1 | 03/11/2018 | ATTEMPT | 13/10/2018 | 2 |
| 1 | 01/11/2018 | INFO | 13/10/2018 | 10 |
| 1 | 22/10/2018 | ARRIVED | 13/10/2018 | 7 |
| 1 | 15/10/2018 | SENT | 13/10/2018 | 2 |
+----+------------+------------+------------+------------+
Notice that when there is no previous code, the date diff is between Udate and Ddate.
Would appreciate any idea.
Thank you.

Answer 1

Well, ISNULL is the way to go here.
Since you also want the average difference, you can use a common table expression to get the difference, and query it to get the average:
First, Create and populate sample data (Please save us this step in your future questions)
-- This would not be needed if you've used ISO8601 for date strings (yyyy-mm-dd | yyyymmdd)
SET DATEFORMAT DMY;
DECLARE #T AS TABLE
(
ID int,
UDate date,
last_code varchar(10),
Ddate date
) ;
INSERT INTO #T (ID, Udate, last_code, Ddate) VALUES
(1, '05/11/2018', 'ACCEPTED', '13/10/2018'),
(1, '03/11/2018', 'ATTEMPT' , '13/10/2018'),
(1, '01/11/2018', 'INFO' , '13/10/2018'),
(1, '22/10/2018', 'ARRIVED' , '13/10/2018'),
(1, '15/10/2018', 'SENT' , '13/10/2018');
The cte:
WITH CTE AS
(
SELECT ID,
Udate,
last_code,
Ddate,
DATEDIFF(
DAY,
ISNULL(
LAG(Udate) OVER(PARTITION BY ID ORDER BY Udate),
Ddate
),
UDate
) As Difference
FROM #T
)
The query:
SELECT *, AVG(Difference) OVER(PARTITION BY ID) As AverageDifference
FROM CTE;
Results:
ID Udate last_code Ddate Difference AverageDifference
1 15.10.2018 SENT 13.10.2018 2 4
1 22.10.2018 ARRIVED 13.10.2018 7 4
1 01.11.2018 INFO 13.10.2018 10 4
1 03.11.2018 ATTEMPT 13.10.2018 2 4
1 05.11.2018 ACCEPTED 13.10.2018 2 4