How can I populate a table in one month increments based on a date

Question

I need to load a table based on the entry date from another table being between the 1st day of a month and the last day of the month. I need to decrement the load to be 1 month's worth of records 1 month from the current month, then 2 months from the current month and so so.

I've tried CTE, using a cursor, and manually changing the datediff parameters. This last method works, but I would like to load the table dynamically

Declare @StartDt as datetime = (SELECT FORMAT(DATEADD(s,0,DATEADD(mm, DATEDIFF(m,0,GETDATE())-1,0)),'MM/dd/yyyy'));

Declare @endDt as datetime = EOMONTH(@StartDt);

SELECT  useruid, 
    userfullname,
    sum(totaltime)SumTime,
    @StartDt as StartDate, 
    @EndDt as EndDate

FROM Table_1.tbl
WHERE ENTRYDATE between @StartDt  and @EndDt
group by useruid, USERFULLNAME;

I am inserting this into another table. I don't have an issue doing that by manually changing -1 in the Startdt variable to -2 and so on. I want to know if I can have it change dynamically while loading the other table.

Again, I have no issues loading the table. I just want to know if there is a more efficient way to do so as I have to go back 100 months.

Answer 1

One method is a recursive CTE:

with dates as (
      select datefromparts(year(getdate()), month(getdate()), 1) as dte
      union all
      select dateadd(month, -1, dte)
      from dates
      where dte > ?  -- how far back do you want to go?
     )
select t.useruid, t.userfullname, sum(t.totaltime) as SumTime,
       d.dte as StartDate, 
       dateadd(month, 1, d.dte) as EndDate
from dates d JOIN
     Table_1.tbl t
     on t.ENTRYDATE >= d.dte AND
        t.ENTRYDATE < dateadd(month, 1, d.dte) 
group by t.useruid, t.USERFULLNAME, d.dte;

You do not need a (an explicit) loop or a cursor.

Note: You seem to be subtracting one second from the date for some reason. I don't understand why. You may need this logic for end date, but it is better to use the inequalities that are in the on clause (so you don't have to worry about milliseconds, for instance).

Answer 2

You can use dynamic SQL to write your @StartDt declare statement on the fly.

I put your code in the while loop which will run 100 times. (go back 100 months):

declare @int int = 0
while @int > -100
begin
set @int= @int -1
declare @str varchar(max)
set @str = 'declare @StartDt as datetime = (SELECT FORMAT(DATEADD(s,0,DATEADD(mm, DATEDIFF(m,0,GETDATE())'+cast(@int as varchar(10))+',0)),''MM/dd/yyyy''))'
execute (@str)

Declare @endDt as datetime = EOMONTH(@StartDt)

SELECT  useruid, 
    userfullname,
    sum(totaltime)SumTime,
    @StartDt as StartDate, 
    @EndDt as EndDate

FROM Table_1.tbl
WHERE ENTRYDATE between @StartDt  and @EndDt
group by useruid, USERFULLNAME
end