How do I create a SQL loop that generates future dates based on different frequencies and intervals for each item until a specified end date?

I have a data set that appears like this:

next_generation_date
procedure
interval
frequency

2021-01-17 00:00:00.000
Clean Restroom
1
day

2021-01-17 00:00:00.000
Vacuum
2
week

2021-02-01 00:00:00.000
Inspect Fire Extinguisher
3
month

2021-10-01 00:00:00.000
Test Generator
4
year

My goal is to generate multiple date rows for each procedure by going off of the next_generation_date, interval and frequency columns until a specified end date. This specified end date would be the same date throughout the entire table. In this example, let's make the specified end date 2025-12-31. My end goal is for the table to appear similar to the below. Please note since this is an example, I didn't include every row as to avoid having hundreds of rows listed below.

next_generation_date
procedure
interval
frequency

2021-01-17 00:00:00.000
Clean Restroom
1
day

2021-01-18 00:00:00.000
Clean Restroom
1
day

2021-01-19 00:00:00.000
Clean Restroom
1
day

2021-01-20 00:00:00.000
Clean Restroom
1
day

2021-01-17 00:00:00.000
Vacuum
2
week

2021-01-31 00:00:00.000
Vacuum
2
week

2021-02-14 00:00:00.000
Vacuum
2
week

2021-02-28 00:00:00.000
Vacuum
2
week

2021-02-01 00:00:00.000
Inspect Fire Extinguisher
3
month

2021-05-01 00:00:00.000
Inspect Fire Extinguisher
3
month

2021-08-01 00:00:00.000
Inspect Fire Extinguisher
3
month

2021-11-01 00:00:00.000
Inspect Fire Extinguisher
3
month

2021-10-01 00:00:00.000
Test Generator
4
year

2025-10-01 00:00:00.000
Test Generator
4
year

To summarize the above table, future "Clean Restroom" dates are each day, "Vacuum" dates are every two weeks, "Inspect Fire Extinguisher" dates are every three months, and "Test Generator" dates are every four years.

Below are two of my most recent attempts in Microsoft SQL Server Management Studio. The first attempt creates the loop but my procedures only increase by an interval of 1, not the unique interval found within the interval column. My second attempt generates the next next_generation_date with the c

I recommend using a Calendar table. Aaron Bertand has a blog series about them

For this code sample, I'll be using my calendar table. It's quite a bit of code so this is a link to my GitHub

For the solution, I'll narrow the range from start to end.
To calculate the interval I'll use the helper offset columns.

Current_offset - starting_offset % interval = 0

That's a logic shared for all frequency types. But some frequencies require additional conditions. For example with the WEEK frequency - the offset applies to a whole week, but we want to limit it to the same weekday (e.g. Sunday with Sunday). Similar for the other frequencies.

Complete solution (relies on the existence of the Calendar table)

DROP TABLE IF EXISTS #RegularTask
CREATE TABLE #RegularTask
(
    StartDate date
    , EndDate date 
    , RegularProcedure varchar(50)
    , interval tinyint
    , frequency varchar(10)
)

INSERT INTO #RegularTask (StartDate, EndDate, RegularProcedure, interval, frequency)
VALUES 
  (DATEFROMPARTS(2021,10,01), DATEFROMPARTS(2025,12,31), 'Test Generator', 4, 'YEAR')
, (DATEFROMPARTS(2021,02,01), DATEFROMPARTS(2025,12,31), 'Inspect Fire Extinguisher', 3, 'MONTH')
, (DATEFROMPARTS(2021,01,17), DATEFROMPARTS(2025,12,31), 'Vacuum', 2, 'WEEK')
, (DATEFROMPARTS(2021,01,17), DATEFROMPARTS(2025,12,31), 'Clean Restroom', 1, 'DAY')

;WITH startPoint
AS
(
    SELECT 
        rt.StartDate
      , rt.EndDate
      , rt.RegularProcedure
      , rt.interval
      , rt.frequency
      , c.YearNum
      , c.MonthNum
      , c.DayOfMonthNum
      , c.DayOfWeekNum
      , c.DayOffset
      , c.WeekOffset
      , c.MonthOffset
    FROM #RegularTask AS rt
    JOIN dbo.Calendar AS c
        ON rt.StartDate = c.DateVal
)
SELECT
      c.DateVal
    , sp.RegularProcedure
    , sp.interval
    , sp.frequency
FROM dbo.Calendar AS c
JOIN startPoint AS sp
    ON c.DateVal >= sp.StartDate
    AND c.DateVal <= sp.EndDate
    AND 
    (
        (
            sp.frequency = 'DAY' 
            AND (c.DayOffset - sp.DayOffset) % sp.interval = 0
        )
        OR 
        (
            sp.frequency = 'WEEK'
            AND (c.WeekOffset - sp.WeekOffset) % sp.interval = 0
            AND c.DayOfWeekNum = sp.DayOfWeekNum
        )
        OR 
        (
            sp.frequency = 'MONTH'
            AND (c.MonthOffset - sp.MonthOffset) % sp.interval = 0
            AND c.DayOfMonthNum = sp.DayOfMonthNum
            
        )
        OR 
        (
            sp.frequency = 'YEAR'
            AND (c.YearNum - sp.YearNum) % sp.interval = 0
            AND c.MonthNum = sp.MonthNum
            AND c.DayOfMonthNum = sp.DayOfMonthNum
        )
    )

StackExchange Database Administrators Q#312530, answer score: 5