根据间隔（月/季）查找下一个日期

Question

CREATE TABLE @Temp(
    Date datetime,
    Flag bit)

@Temp table as data from 01-04-2019  to  31-04-2020  (366 records) and flag=0

DECLARE startdate date='12-04-2019', interval  int  =1 OR 3

Expected Result: If interval is 1 month, based on startdate, flag to be updated as 1 below records and rest for 0

    date         flag
    01-04-2019    0
    .             0
    .             0
    12-05-2019    1
    .             0
    .             0  
    12-06-2019    1
    .             0
    .             0
    12-07-2019    1
    ..            0
    31-04-2020    0

If interval is 3 month, flag to be updated as 1 for quarter

      date        flag
    01-04-2019    0
    .             0
    .             0
    12-07-2019    1
    .             0
    .             0  
    12-10-2019    1
    .             0
    .             0
    12-01-2020    1
    .             0
    31-04-2020    0

I am stuck trying get the result. I am using SQL Server 2017.

Answer 1

Took me awhile to realize that your dates were in different format than I'm used to.

I would go with a common table expression as opposed to a cursor. Besides that, consider the month difference between any given date and the start date. Take the modulo of that difference (that's what the '%' symbol is). If it's 0, then your interval has been hit and so activate your flag.

declare 
    @startdate date = '04.01.2019',
    @interval int = 3; -- or 1, or whatever

with

    dates as (

        select      @startdate dt, 1 flag

        union all
        select      ap.nextDt, 

                    flag = 
                        case
                        when day(ap.nextDt) <> day(@startdate) then 0 
                        when (month(ap.nextDt) - month(@startdate)) % @interval = 0 then 1
                        else 0 
                        end

        from        dates
        cross apply (select nextDt = dateadd(day,1,dt)) ap
        where       dt <= dateadd(year,1,@startdate)

    )

    select    *           
    from      dates 
    option    (maxrecursion 367)