Add one month until it is greater than some value - sql

Question

I have start date & Mdate as columns in table, i want to do something like below in SQL
Add 1 month to Start_Date until start date > Mdate
I tried using while and if concepts, but no luck.
DECLARE #MIGRATIONDATE DATE, #STRT_DATE DATE, #NEXTD DATE
SET #MIGRATIONDATE =20140725
SET #STRT_DATE = 20140521
SELECT WHILE ( #STRT_DATE > #MIGRATIONDATE)
BEGIN
DATEADD(MM,1,#STRT_DATE))
END
appreciate if you can guide me on this?

Answer 1

I do not know if you would like to get one row of that table. If you want to do it for each row, then you should wrap it in a function and use CROSS APPLY.
declare
#startdate datetime,
#enddate datetime
set #startdate = '20140101'
set #enddate = '20150101'
;WITH date_range (thedate) AS (
select #startdate
UNION ALL SELECT DATEADD(MONTH, 1, thedate)
FROM date_range
WHERE DATEADD(MONTH, 1, thedate) <= #enddate
)
SELECT thedate FROM date_range
If you wanted in a function:
CREATE FUNCTION [dbo].[ExplodeDates](#startdate datetime, #enddate datetime)
returns table as
return (
WITH date_range (thedate) AS (
select #startdate
UNION ALL SELECT DATEADD(DAY, 1, thedate)
FROM date_range
WHERE DATEADD(DAY, 1, thedate) <= #enddate
)
SELECT thedate FROM date_range
);
SELECT Id,thedate
FROM Table1 T1
CROSS APPLY [dbo].[ExplodeDates](T1.StartDate,T1.EndDate)

Answer 2

select
case when start_date<Mdate then dateadd(mm,1,start_date) else start_date end
from yourTable
suppose start_date is 20140721 and MigrationDate is 20140525 then it returns you with gives you accepted result
select case
when convert( date,'20140721') <convert(date,'20140525')
then dateadd(mm,1,'20140721') else '20140721' end

Answer 3

How about (SQL Fiddle):
SELECT DATEADD(month,
DATEDIFF(month, Start_Date, Mdate) + 1,
Start_Date) AS NEW_START_DATE
FROM MyTable;
If there is a possibility of the Start_Date being greater than or equal to Mdate then use the following (SQL Fiddle):
SELECT Start_Date AS OLD_START_DATE,
CASE WHEN DATEDIFF(month, Start_Date, Mdate) > 0
THEN DATEADD(month, DATEDIFF(month, Start_Date, Mdate) + 1, Start_Date)
ELSE Start_Date END AS NEW_START_DATE,
Mdate
FROM MyTable;