Is that possible to write the below query using CTE recursion? - sql

Question

create table #temp (date date)
declare #X date
set #X = '2016-7-01'
declare #Y date
set #Y = cast (getdate() as date)
while(#X<=#Y)
begin
if (datename(WEEKDAY,#X) = 'Sunday')
insert into #temp values (#X)
set #X = cast(((cast(#X as datetime))+1)as date)
continue
end
select * from #temp
drop table #temp
Is that possible to write the above query using CTE recursion?

Answer 1

You can use a CTE to create a numbers table. You can then use the numbers table to get your dates like so:
Declare #Startdate Datetime = '2016-07-01'
Declare #EndDate Datetime = '2016-08-29'
;with
N0 as (SELECT 1 as n UNION ALL SELECT 1)
,N1 as (SELECT 1 as n FROM N0 t1, N0 t2)
,N2 as (SELECT 1 as n FROM N1 t1, N1 t2)
,N3 as (SELECT 1 as n FROM N2 t1, N2 t2)
,N4 as (SELECT 1 as n FROM N3 t1, N3 t2)
,nums as (SELECT ROW_NUMBER() OVER (ORDER BY (SELECT 1)) as num FROM N4)
SELECT DATEADD(day,num-1,#startdate) as thedate
FROM nums
WHERE num <= DATEDIFF(day,#startdate,#enddate) + 1
and datename(WEEKDAY, DATEADD(day,num-1,#startdate)) = 'Sunday'
Each table (N0 to nums) effectively multiplies the number of rows in the previous 'table', so you end up with 65,536 rows of numbers in nums (you can do less or more by adding or removing table NX as required). Then, use the numbers table to add days to your start date(SELECT DATEADD(day,num-1,#startdate) as thedate) , where the dates returned are in your date range, and the weekday is Sunday.
Also, because the numbers in nums start at 1, we use nums-1 in our select, so as to avoid skipping over the first date in our series, effectively giving us DATEADD(day, 0, #startdate) in our first row.

Answer 2

You can try something like explained here: http://blog.sqlauthority.com/2009/12/29/sql-server-get-date-of-all-weekdays-or-weekends-of-the-year/
DECLARE #StartDate DATETIME
DECALRE #EndDate DATETIME
SET #StartDate = '2016-07-01'
SET #EndDate = GETDATE()
;WITH cte AS (
SELECT
1 AS DayID,
#StartDate AS FromDate,
DATENAME(dw, #StartDate) AS Dayname
UNION ALL
SELECT
cte.DayID + 1 AS DayID,
DATEADD(d, 1, cte.FromDate),
DATENAME(dw, DATEADD(d, 1, cte.FromDate)) AS Dayname
FROM cte
WHERE DATEADD(d, 1, cte.FromDate) < #EndDate
)
SELECT FromDate AS Date, Dayname
FROM cte
WHERE Dayname IN ('Sunday')
I'm thinking there's be a more efficient way to do this by using a numbers table and a factor of 7 (depending on how many dates you need to get, possible pre-calc the first sunday from your start date, then join all numbers that are factors of 7 from a numbers table), but the above works well enough also.

Answer 3

;with cte
as
(
select getdate() as datee
union all
select dateadd(day,1,datee)
from cte
where datediff(day,getdate(),datee)<100
)
select * from cte
where datename(WEEKDAY,datee) = 'Sunday'
You will hit max recursion limit as well ,to avoid..that use something like below..
option ( MaxRecursion 0 )
I would solve this using numbers table if i am not constrained by the need to use recursive cte ,which is also way faster than Recursive cte..
select
dateadd(day,n,getdate()) as datee
from numbers
where n<100 and datename(weekday,dateadd(day,n,getdate()))='sunday'
To learn why you need numbers table,check this link..https://dba.stackexchange.com/questions/11506/why-are-numbers-tables-invaluable