Get range/count of days based off a single date field

Question

I have requirement where i will need to get the number of days a role an employee was on.

Scenario 1

   EmployeeId    role   effectiveFrom
      1           A      1-Jan-2021
      1           B      15-Jan-2021

No further roles are available for the month of Jan for role A therefore the number of days for role A would be 14.

Scenario 2

   EmployeeId    role   effectiveFrom
      1           A      1-Jan-2021

No further roles are available for the month of Jan therefore the number of days for role A would be 31 ie the entire month of January. For the month of February i would expect to get 28 as the role would be effective for the entire month of february as well.

Scenario 3

   EmployeeId    role   effectiveFrom
      1           A      1-Jan-2021
      1           B      15-Jan-2021
      1           A      25-Jan-2021

To get the number of days for role A the logic would be

• 1 to 15th is 14 days.
• 25th to 31st(31st of Jan) would be 6 days.
• 14 + 6 = 20 days

The query i have come up with so far is this,

SELECT 
    DATEDIFF(MAX(effectiveFrom),
            IF(MIN(effectiveFrom) = MAX(effectiveFrom),
                MIN(effectiveFrom),
                MIN(effectiveFrom))) + 1 daysWorked
FROM
    EmployeeRoles
WHERE grade = 'A'  
GROUP BY `employeeId`,effectiveFrom;

which would only give the result as 1 day for Scenario 1. Could someone guide me on the practical way of handling the scenarios. I have looked at loops, window functions but i am at a loss on the best way to proceed.

dbfiddle

Answer 1

When scenario2 has 31 days from 1-jan, until the end of the month, I would suspect that from 25-jan, until the end of the month, is 7 days, and not 6, as you write in scenario3.

The number of days, using above calculation:

SELECT
   employeeID,
   grade,
   effectiveFrom,
   DATEDIFF(COALESCE(LEAD(effectiveFrom) 
                             OVER (PARTITION BY employeeID, grade ORDER By effectiveFrom),
                     DATE_ADD(LAST_DAY(effectiveFrom),INTERVAL 1 DAY)),
            effectiveFrom) as '#Days'
FROM EmployeeRole;

This can be grouped, and summed giving:

SELECT
   employeeID,
   grade,
   SUM(`#Days`)
FROM (

SELECT
   employeeID,
   grade,
   effectiveFrom,
   DATEDIFF(COALESCE(LEAD(effectiveFrom) 
                            OVER (PARTITION BY employeeID, grade ORDER By effectiveFrom),
                     DATE_ADD(LAST_DAY(effectiveFrom),INTERVAL 1 DAY)),
            effectiveFrom) as '#Days'
FROM EmployeeRole
) x
GROUP BY
  employeeID,
   grade;

output:

employeeID	grade	SUM( #Days )
1	A	14
1	B	17
2	A	31
3	A	21
3	B	10

see: DBFIDDLE

EDIT: The results were incorrect because the next effectiveFrom date was determined using OVER (PARTITION BY employeeID ORDER By effectiveFrom) . this is not correct, because the grade should be taken into account too.

I corrected it to OVER (PARTITION BY employeeID, grade ORDER By effectiveFrom)

PS I also corrected this in the piece above the EDIT!

see: DBFIDDLE