Is there way to add date difference values we get to the date automatically? - sql

Question

What I was trying to do is I have two dates and using DateDiff to get a difference between dates. For example, I Have planned Start Date and actual start Date and I got the difference between this date is 5, now I want to add this day to the Finish date.
If my Finish date is not what I assumed, but behind, then that difference we got I want to add and want to find next finish date because we are behind so next upcoming dates.
Sum (DATEDIFF(day, sa.PlannedStartDate, sa.ActualStartDate)) OVER
(Partition
By ts.Id)as TotalVariance,
Case when (Sum (DATEDIFF(day, sa.PlannedStartDate, sa.ActualStartDate))
OVER
(Partition By ts.Id) >30) then 'Positive' end as Violation,
DATEADD (day, DATEDIFF(day, sa.PlannedStartDate, sa.ActualStartDate))as
Summar violations,
If the activity 1 - planned Start date is 8/21/2019 but the actual start date is 9/21/2019, in this case we are behind 30 days.
Now the next activity will be delayed, so I want to add this difference to the next activity.
If the second activity planned Start date was 08/25/2019, but because of the delay of activity 1 the start date will change for second activity, in this case I want to find that new date.
Activity PlannedStartdate ActualStartDate Variance NewPlannedstartdate
Activity 1 8/21/2019 9/21/2019 30
Acivity 2 8/26/2019 null 9/26/2019

Answer 1

Here's an example you can run in SSMS:
-- CREATE ACTIVITY TABLE AND ADD SOME DATA --
DECLARE #Activity TABLE ( ActivityId INT, PlannedStart DATE, ActualStart DATE );
INSERT INTO #Activity (
ActivityId, PlannedStart, ActualStart
)
VALUES
( 1, '08/21/2019', '08/27/2019' ), ( 1, '08/26/2019', NULL ), ( 1, '09/14/2019', NULL );
Query #Activity to see what's in it:
SELECT * FROM #Activity ORDER BY ActivityId, PlannedStart;
#Activity content:
+------------+--------------+-------------+
| ActivityId | PlannedStart | ActualStart |
+------------+--------------+-------------+
| 1 | 2019-08-21 | 2019-08-27 |
| 1 | 2019-08-26 | NULL |
| 1 | 2019-09-14 | NULL |
+------------+--------------+-------------+
Query #Activity to factor the new starting dates:
;WITH Activity_CTE AS (
SELECT
ROW_NUMBER() OVER ( ORDER BY PlannedStart ) AS Id,
ActivityId, PlannedStart, ActualStart, DATEDIFF( dd, PlannedStart, ActualStart ) Delayed
FROM #Activity
WHERE
ActivityId = #ActivityId
)
SELECT
ActivityId,
PlannedStart,
ActualStart,
DATEADD( dd, Delays.DaysDelayed, PlannedStart ) AS NewStart
FROM Activity_CTE AS Activity
OUTER APPLY (
SELECT CASE
WHEN ( Delayed IS NOT NULL ) THEN Delayed
ELSE ISNULL( ( SELECT TOP 1 Delayed FROM Activity_CTE WHERE Id < Activity.Id AND Delayed IS NOT NULL ORDER BY Id DESC ), 0 )
END AS DaysDelayed
) AS Delays
ORDER BY
PlannedStart;
Returns
+------------+--------------+-------------+------------+
| ActivityId | PlannedStart | ActualStart | NewStart |
+------------+--------------+-------------+------------+
| 1 | 2019-08-21 | 2019-08-27 | 2019-08-27 |
| 1 | 2019-08-26 | NULL | 2019-09-01 |
| 1 | 2019-09-14 | NULL | 2019-09-20 |
+------------+--------------+-------------+------------+
The real "magic" here is this line:
ELSE ISNULL( ( SELECT TOP 1 Delayed FROM Activity_CTE WHERE Id < Activity.Id AND Delayed IS NOT NULL ORDER BY Id DESC ), 0 )
It's checking for any prior records to itself that has a delay. If none are found, it returns 0. This value is then used to add days to the PlannedStart date to determine the NewStart date. The ORDER BY is of particular note too. Sorting in a DESC order ensures we get the "closest" delay prior to the current row.
Using a CTE in this way also takes into account the idea that the delay may not happen on the very first record (e.g., say the 08/26 planned was delayed instead of 08/21). It conveniently gives us a subtable to query against in our OUTER APPLY.
This is what you would see if you included all columns on the CTE's SELECT:
+----+------------+--------------+-------------+---------+-------------+
| Id | ActivityId | PlannedStart | ActualStart | Delayed | DaysDelayed |
+----+------------+--------------+-------------+---------+-------------+
| 1 | 1 | 2019-08-21 | 2019-08-27 | 6 | 6 |
| 2 | 1 | 2019-08-26 | NULL | NULL | 6 |
| 3 | 1 | 2019-09-14 | NULL | NULL | 6 |
+----+------------+--------------+-------------+---------+-------------+
Because the very first record is the only record with a delay, its delay of 6 days persists through each of the following records.