IF statment in SQL - sql-server

Question

I have a table_changes (Id,stard_date,end_date) and I want to add two columns rank_end_date and new_end_date.
The problem I have in my data is that not always there is continuousness (in the month level, the day in the month is not in my intrest) between end_date and the start_date coming just after it (see example 1) so I need to "strech" end_date in some cases so there will be continuousness at the level of the month.
For example 1, the new_end_date is 1/2/2015 and doesn't have to be 28/2/2015. If the end_date in rank 1 is sooner than 31/12/2015 strech it to 31/12/9999.
Some Examples:
Ex1:
Id --start date --end_date --rank_end_date new_end_date
111 01/01/1970 1/1/1980 2 1/2/2015
111 01/03/2015 31/12/9999 1 31/12/9999
Ex2:
Id --start_date --end_date --rank_end_date new_end_date
111 01/01/1970 1/1/1980 1 31/12/9999
Ex3:
Id --start_date --end_date --rank_end_date new_end_date
111 01/01/1970 1/1/1980 2 01/05/1990
111 01/05/1990 31/12/1995 1 31/12/9999
Ex4:
Id --start_date --end_date --rank__end_date new_end_date
111 01/03/2015 31/12/9999 1 31/12/9999
Ex5:
Id --start_Date --end_date --rank__end_date new_end_date
111 01/02/2015 31/5/2015 2 01/5/2015
111 01/06/2015 31/12/9999 1 31/12/9999
the syntax should be something like this but I don't know how to write those IF statements in SQL:
if rank_end_date ==2 then new_end_date == 1/Month(start_date(rank_end_date - 1)) - 1 /2015
if rank_end_date ==1 then new_end_date == 31/12/2015
else new_end_date = end_date
Select [Id],[StartDate],[EndDate],
Rank_End_Date, case
when t.Rank_End_Date = (2) **then
CAST(CAST(Year([StartDate]) AS varchar) + '-' + CAST(Month([StartDate]) AS varchar) + '-' +
--How to do I choose the Start_Date from the record with Rank==1? It is selecting
the start date from the record with rank==2 ofcourse.
CAST(Day ([EMER_StartDate]) AS varchar) AS DATE)
when t.Rank_End_Date = (1) then '9999-12-31'
else t.[EMER_EndDate] end As New_End_Date
from (
Select [Id],[StartDate],[EndDate],
Rank() OVER (PARTITION BY [Id] order by [EndDate] desc) as Rank_End_Date
from [dbo].[Changes]
) t
Could anybody help in achieving the result?

Answer 1

If I've understood your question right, and you can only have values in rank_end_date of 1 or 2 then something like this query should give you the answer you're looking for. Either way, the LEAD (or LAG function if you sort the records ascending) will allow you to fetch the value from a different record.
SELECT ID
, start_date
, end_date
, rank_end_date
, CASE WHEN rank_end_date = 1 THEN
CASE WHEN end_date < '31/12/2005' THEN '31/12/9999' ELSE end_date END
WHEN rank_end_date = 2 THEN LEAD(start_date,1) OVER(ORDER BY ID, rank_end_date DESC)
END AS new_end_date
FROM dbo.Changes

Answer 2

You can't use LEAD OR LAG functions in SQL Server 2008, so you can try this solution.
with CTE as
(
Select [Id] as ID,[StartDate] as StartDate,[EndDate] as EndDate,
ROW_NUMBER() OVER (PARTITION BY [Id] order by [StartDate] DESC) as rn_Start_Date
from [dbo].[Changes]
)
Select C1.[Id] , C1.[StartDate], C1.[EndDate], C1.rn_Start_Date as Rank_end_date,
ISNULL(DATEADD(MONTH, DATEDIFF(MONTH, 0, C2.[StartDate])-1, 0), cast('9999-12-31' as DATE)) As New_End_Date
From CTE C1
LEFT JOIN CTE C2 ON C1.[ID] = C2.[ID] AND C1.Rn_Start_Date = C2.Rn_Start_Date + 1