How do i calculate the dates in this scenario

Question

I have a situation here and i am not able to think through it
there are 4 date columns in a table and i need to do something like below
First Record start date is the start date, and end date is the next start date -1
Next start date is next start date, and end date is the first end date of the record set of the end dates
Next start date pervious end date + 1 and end date is the current end date
Next start date is the current date and next end date is the current end date
for example lets say we have a table like below
CustomerID CustomerType CustomerEffDt   CustomerTrmDt CustNationalBgnDt CustNationalEndDt
1                  National           2013-08-05        9999-12-31      
2010-11-08            2011-11-02
Now my question is, we would have to calculate the customer effective start and begin dates and national begin and end dates based on the above criteria
we would have to create 4 rows based on the above scenario
I am going mad and not able to think through this
Can somebody help me
Thanks

Answer 1

Declare @Example table (CustomerID int, CustomerEffDt date, CustomerTrmDt date, CustNationalBgnDt date, CustNationalEndDt date, otherstuff varchar(99))
insert @Example Select 1, '3/19/2014', '9999-12-31', '3/19/2015', '2/19/2016', 'first'
UNION Select 2, '3/19/2014', '9999-12-31', '4/19/2014', '5/28/2014', 'second'
UNION Select 3, '2013-08-05', '9999-12-31', '2010-11-08', '2011-11-02', 'first example by OP'
Select CustomerID,Num
, case When Num = 1 Then '1/1/1900'
when NUM = 2 then CustNationalBgnDt
when NUM = 3 then DateAdd(day, 1, CustNationalEndDt )
when NUM = 4 then CustomerEffDt End as startdate
, case When NUM = 1 then DateAdd(day, -1, CustNationalBgnDt)
when NUM = 2 then CustNationalEndDt
when NUM = 3 then Dateadd(Day, -1, CustomerEffDt)
when NUM = 4 then CustomerTrmDt End as EndDate
From @EXAMPLE
Cross Apply (Select 1 as Num UNION Select 2 UNION Select 3 UNION Select 4) as Numbers
order by CustomerID, Num
/* --Your result
1 1 1900-01-01 2015-03-18
1 2 2015-03-19 2016-02-19
1 3 2016-02-20 2014-03-18
1 4 2014-03-19 9999-12-31
2 1 1900-01-01 2014-04-18
2 2 2014-04-19 2014-05-28
2 3 2014-05-29 2014-03-18 ---***
2 4 2014-03-19 9999-12-31 --****
3 1 1900-01-01 2010-11-07
3 2 2010-11-08 2011-11-02
3 3 2011-11-03 2013-08-04
3 4 2013-08-05 9999-12-31
CustomerID StartDate EndDate
1 1900-01-01 2014-03-18
1 2014-03-19 2015-03-18
1 2015-03-19 2016-02-19
1 2016-02-20 9999-12-31
2 1900-01-01 2014-03-18
2 2014-03-19 2014-04-18
2 2014-04-19 2014-05-28
2 2014-05-29 9999-12-31
3 1900-01-01 2010-11-07
3 2010-11-08 2011-11-02
3 2011-11-03 2013-08-04
3 2013-08-05 9999-12-31
;with mycte as
select CustomerID, dt,col,row_number() Over(partition by CustomerID Order by dt) rn
,row_number() Over(partition by CustomerID Order by dt DESC) rn2 from @Example
cross apply (
values( CustomerEffDt,'CustomerEffDt'), (CustomerTrmDt,'CustomerTrmDt')
, (CustNationalBgnDt,'CustNationalBgnDt'), (CustNationalEndDt,'CustNationalEndDt'))
d(dt,col)
Select m1.CustomerID,
Case when m1.rn=1 Then '1/1/1900'
When m2.col='CustNationalEndDt' Then dateadd(day,1,m2.dt)
Else m2.dt end as StartDate,
Case
when m1.rn2=1 Then m1.dt
When m1.col='CustNationalEndDt' Then m1.dt
else dateadd(day,-1,m1.dt) End
as EndDate
From mycte m1 LEFT Join mycte m2 on m1.CustomerID=m2.CustomerID AND m1.rn=m2.rn+1
Order by m1.CustomerID, m1.dt