Generate Dates recursively in SQL Server - sql

Question

I have some dates I want to calculate which is currently done over several subqueries. Each subsequent subquery uses the result (a date) of the previous query in its calculation. E.g.
DECLARE #Date DATE = '20170101'
SELECT #foo1 = (SELECT TOP 1 dbo.DateFunction(DateField)
FROM [DateTable]
WHERE DateField <= #Date
ORDER BY DateField DESC)
SELECT #foo2 = (SELECT TOP 1 dbo.DateFunction(DateField)
FROM [DateTable]
WHERE DateField <= #foo1
ORDER BY DateField DESC)
....
SELECT #fooN = (SELECT TOP 1 dbo.DateFunction(DateField)
FROM [DateTable]
WHERE DateField <= #fooNMinus1
ORDER BY DateField DESC)
Is it possible (perhaps using CTE) to make a recursive query to achieve this for a specified number of times?

Answer 1

Weeks are almost always 7 days, so you can get the first one and then just add seven days. If so:
WITH dates as (
SELECT MAX(dbo.DateFunction(DateField)) as dte, 1 as counter
FROM [DateTable]
WHERE DateField <= #Date
UNION ALL
SELECT DATEADD(DAY, 7, dte), counter + 1
FROM dates
WHERE counter < #n
)
SELECT dte
FROM dates;

Answer 2

You can use small tally table as below
Declare #d1 date = '2017-01-01'
Declare #d2 date = '2017-12-31'
select top (datediff(day, #d1, #d2)+1) dt = DateAdd(day, Row_Number() over (order by (Select NULL))-1, #d1)
from master..spt_values s1, master..spt_values s2
Or custom tally tables
;with num as
( select * from (values (1),(1),(1),(1),(1),(1),(1),(1),(1),(1)) v(n) )
, n1 as (select n1.* from num n1, num n2, num n3, num n4) --numbers generation
select top (datediff(day, #d1, #d2)+1) dt = DateAdd(day, Row_Number() over (order by (Select NULL))-1, #d1)
from n1

Answer 3

Yes, you can use a recursive query. Since top and aggregates are not allowed in the recursive part, you can use the row_number() function instead.
Declare #date date = cast(getdate() as date), #n int = 10
declare #DateTable table (DateField date)
insert into #DateTable values ('2017-05-01'),('2017-05-02'),('2017-05-03'),('2017-05-04'),('2017-05-05'),('2017-05-06'),('2017-05-07'),('2017-05-08'),('2017-05-09'),('2017-05-10'),
('2017-05-11'),('2017-05-12'),('2017-05-13'),('2017-05-14'),('2017-05-15'),('2017-05-16'),('2017-05-17'),('2017-05-18'),('2017-05-19'),('2017-05-20')
;with date_rte as (
select top 1 dbo.DateFunction(DateField) datefield, 0 recursions, cast(1 as bigint) rn
from #dateTable
where datefield <= #date
order by datefield desc
union all
select dbo.DateFunction(DateField), recursions+1, ROW_NUMBER() over (order by d.datefield desc)
from #datetable d
join date_rte r on d.DateField <= r.datefield
where recursions < #n and rn = 1
)
select datefield
from date_rte
where rn=1 and recursions = #n