Calculate Date difference between multiple rows SQL

Question

I need to calculate the date difference between multiple rows. The scenario is I have a vehicle that can do inspections throughout the month as well as when the vehicle is assigned to a different project. I want to calculate that how many days that a vehicle is assigned to the project per month or previous month. I have tried multiple ways and I can't get even closer. I am relatively new to stack overflow. Apologies if anything is missing. Please let me know if this can be done. Thank you.

All the columns are in one single table if that helps. Please let me know the query on how to achieve this

I am using SQL server 2017.

Original Data

Expected Output

Answer 1

I am not proud of this solution, but I think it works for you. My approach was to create a table of days and then look at which project the vehicle was assigned to each day. Finally, aggregate by month and year to get the results. I had to do this as a script since you can use aggregate functions in the definitions of recursive CTEs, but you may find a way to do this without needing a recursive CTE.

I created a table variable to import your data so I could write this. Note, I added an extra assignment to test assignments that spanned months.

DECLARE @Vehicles AS TABLE
(
    [VehicleID]      INT     NOT NULL,
    [Project]        CHAR(2) NOT NULL,
    [InspectionDate] DATE    NOT NULL
);

INSERT INTO @Vehicles
(
    [VehicleID],
    [Project],
    [InspectionDate]
)
VALUES
(1, 'P1', '2021-08-20'),
(1, 'P1', '2021-09-05'),
(1, 'P2', '2021-09-15'),
(1, 'P3', '2021-09-20'),
(1, 'P2', '2021-10-10'),
(1, 'P1', '2021-10-20'),
(1, 'P3', '2021-10-21'),
(1, 'P2', '2021-10-22'),
(1, 'P4', '2021-11-15'),
(1, 'P4', '2021-11-25'),
(1, 'P4', '2021-11-30'),
(1, 'P1', '2022-02-05');

DECLARE @StartDate AS DATE, @EndDate AS DATE;

SELECT @StartDate = MIN([InspectionDate]), @EndDate = MAX([InspectionDate])
FROM   @Vehicles;

;WITH [seq]([n])
AS (SELECT 0 AS [n]
    UNION ALL
    SELECT [n] + 1
    FROM   [seq]
    WHERE  [n] < DATEDIFF(DAY, @StartDate, @EndDate)),
      [days]
AS (SELECT DATEADD(DAY, [n], @StartDate) AS [d]
    FROM   [seq]),
      [inspections]
AS (SELECT [VehicleID],
           [Project],
           [InspectionDate],
           LEAD([InspectionDate], 1) OVER (PARTITION BY [VehicleID]
                                           ORDER BY [InspectionDate]
                                          ) AS [NextInspectionDate]
    FROM   @Vehicles),
      [assignmentsByDay]
AS (SELECT [d].[d], [i].[VehicleID], [i].[Project]
    FROM   [days] AS [d]
           INNER JOIN [inspections] AS [i]
               ON [d].[d] >= [i].[InspectionDate]
                  AND [d] < [i].[NextInspectionDate])

SELECT   [assignmentsByDay].[VehicleID],
         [assignmentsByDay].[Project],
         MONTH([assignmentsByDay].[d]) AS [month],
         YEAR([assignmentsByDay].[d]) AS [year],
         COUNT(*) AS [daysAssigned]
FROM     [assignmentsByDay]
GROUP BY [assignmentsByDay].[VehicleID],
         [assignmentsByDay].[Project],
         MONTH([assignmentsByDay].[d]),
         YEAR([assignmentsByDay].[d])
ORDER BY [year], [month], [assignmentsByDay].[VehicleID], [assignmentsByDay].[Project]
OPTION(MAXRECURSION 0);

And the output is:

VehicleID	Project	month	year	daysAssigned
1	P1	8	2021	12
1	P1	9	2021	14
1	P2	9	2021	5
1	P3	9	2021	11
1	P1	10	2021	1
1	P2	10	2021	20
1	P3	10	2021	10
1	P2	11	2021	14
1	P4	11	2021	16
1	P4	12	2021	31
1	P4	1	2022	31
1	P4	2	2022	4