The right way to use CTE - sql

Question

I'm new to Common Table Expressions and I think I need to use one in order to achieve what I require.
If I run the following script -
select MainRentAccountReference,EffectiveFromDate,CollectionDay,NumberOfCollections,DirectDebitTotalOverrideAmount
from DirectDebitApportionment
where id = 1
It would give me the below results -
So for each row that my CTE would return- for each unique MainRentAccountReference - I would want to create a row based on the following criteria.
3 Rows as the NumberOfCollections is set to 3
The following dates on each row - 01/05/18, 01/06/18, 01/07/18 so basically plus one month.
However is the CollectionDate was set to say 10, then I would want the 3 dates to be 10/05/18, 10/06/18, 10/07/18
Finally each row to have a value of DirectDebitTotalOverrideAmount divided by number of NumberOfCollections.
I've been playing about with this and can get no where near the results I'm trying to achieve. Any help would be greatly appreciated. Thanks

Answer 1

You can do this with a recursive CTE
with t as (
select *
from DirectDebitApportionment
where id = 1
),
cte as (
select . . ., , 1 as collection, DirectDebitTotalOverrideAmount / NumberOfCollections as collection_amount
from t
union all
select . . ., , collection + 1, DirectDebitTotalOverrideAmount / NumberOfCollections as collection_amount
from cte
where collection < NumberOfCollections
)
select . . .
from cte;
In some dialects of SQL, you need the recursive keyword.
Also, this can also be accomplished using a numbers table -- and that can be more efficient than the recursive CTE (although recursive CTEs often perform surprisingly well).

Answer 2

This seems to do the trick based on the pointers that Gordon gave me -
with t as (
select MainRentAccountReference,EffectiveFromDate,CollectionDay,NumberOfCollections,DirectDebitTotalOverrideAmount
from DirectDebitApportionment
where id = 1
),
cte as (
select 1 as collection
,t.MainRentAccountReference
,convert(decimal(18,2),DirectDebitTotalOverrideAmount / NumberOfCollections) as collection_amount
,NumberOfCollections
,convert(datetime,DATEFROMPARTS ( DATEPART(YEAR,EffectiveFromDate), DATEPART(MONTH,EffectiveFromDate), CollectionDay )) AS EffectiveFromDate
,CollectionDay
from t
union all
select collection + 1,MainRentAccountReference,collection_amount,NumberOfCollections,DATEADD(M,1,EffectiveFromDate),CollectionDay
from cte
where collection < cte.NumberOfCollections
)
select *
from cte
Order by MainRentAccountReference,collection
;
Gives me the following results -