Sum of age between two dates in Years Month Days format - sql

Question

I need to calculate age in years, months and days format between two dates (DateFrom and DateTo) in a way that:
If day of month in DateFrom is 1st then take it as a whole month
If day of month in DateFrom is not 1st then count days till the end of the month.
Example:
DateFrom='2010-02-01', DateTo='2011-03-11', Age= 1 Years, 1 Months 11 Days
DateFrom='2010-02-02', DateTo='2011-03-11', Age= 1 Years, 1 Months 8 Days
After calculating the age I'd need to sum the ages assuming that month is 30 days - in the above example I'd expect result: 2 Years, 2 Months, 19 Days.

Answer 1

My assumption would be that you have have a table containing "DateFrom" and "DateTo" columns so the query would be something like this:
DECLARE #TotalDiffInDays int = (SELECT AVG(DATEDIFF(DAY, DateFrom, DateTo)) AS [TotalDays] FROM #t)
SELECT #TotalDiffInDays
DECLARE #DaysInMonth int = 30;
DECLARE #DaysInYear int = 365;
SELECT
#TotalDiffInDays / 365 AS AvgYearsDiff,
(#TotalDiffInDays / #DaysInMonth - #TotalDiffInDays / #DaysInYear * 12) AS AvgMonthsDiff,
#TotalDiffInDays - ((#TotalDiffInDays / 365) * #DaysInYear + (#TotalDiffInDays / #DaysInMonth - #TotalDiffInDays / #DaysInYear * 12) * #DaysInMonth) AS AvgDaysDiff
Note that in this case I am using INT division to get correct numbers.
In addition, if you just want to get dates difference in the format you desribed above you can use this query:
SELECT
DATEDIFF(YEAR, DateFrom, DateTo) AS [Years],
-- Add year diff to get corret months diff
DATEDIFF(MONTH, DATEADD(YEAR, DATEDIFF(YEAR, DateFrom, DateTo), DateFrom), DateTo) AS [Months],
-- Add months diff to get correct days diff
DATEDIFF(DAY, DATEADD(MONTH, DATEDIFF(MONTH, DateFrom, DateTo), DateFrom), DateTo) AS [Days]
FROM #t
I hope this will helps.
Yeah, just forgot about your rule of the 1st and not 1st day of the month, you can easily modify those queries with adding the DATEPART(DAY , your_date) function to check if it's a 1st date of the month and perform DETEADD() to apply this logic or just add a value of 1 before AVG.

Answer 2

So basically since this custom date maths, you'd need to implement your own functions to get desired results.
See the TSQL below
CREATE FUNCTION dbo.getDateSum(#d1 varchar(100), #d2 varchar(100))
RETURNS varchar(100)
WITH EXECUTE AS CALLER
AS
BEGIN
DECLARE #y int, #m int, #d int
Select
#d1= REPLACE(REPLACE(REPLACE(#d1,' years, ', '-'),' months, ','-'),' days',''),
#d2= REPLACE(REPLACE(REPLACE(#d2,' years, ', '-'),' months, ','-'),' days','')
Select
#y= CAST(LEFT(#d1,CHARINDEX('-',#d1)-1) AS INT)+CAST(LEFT(#d2,CHARINDEX('-',#d2)-1) AS INT),
#m= CAST(SUBSTRING(#d1,CHARINDEX('-',#d1)+1,LEN(#d1)-CHARINDEX('-',REVERSE(#d1))-CHARINDEX('-',#d1))AS INT)+CAST(SUBSTRING(#d2,CHARINDEX('-',#d2)+1,LEN(#d2)-CHARINDEX('-',REVERSE(#d2))-CHARINDEX('-',#d2)) AS INT),
#d= CAST(LEFT(REVERSE(#d1),CHARINDEX('-',REVERSE(#d1))-1)AS INT)+CAST(LEFT(REVERSE(#d2),CHARINDEX('-',REVERSE(#d2))-1) AS INT)
IF(#d>30)
BEGIN
SET #d=#d%30
SET #m=#m+CAST(#d/30 as INT)
END
IF(#m>30)
BEGIN
SET #m=#m%12
SET #y=#y+CAST(#m/12 as INT)
END
RETURN (cast(#y as varchar)+ ' years, ' + cast(#m as varchar) +' months, '+ cast(#d as varchar) + ' days' );
END
go
CREATE FUNCTION dbo.getDateDiff(#df date, #dt date)
RETURNS varchar(100)
WITH EXECUTE AS CALLER
AS
BEGIN
declare #y int, #m int, #d int
Select #y= YEAR(#dt)- YEAR(#df),#m= MONTH(#dt)- MONTH(#df),#d=CASE WHEN DAY(#df)=1 THEN DAY(#dt)- DAY(#df)+1 ELSE DAY(#dt)- DAY(#df) -1 END
If (#d<0)
BEGIN
Set #m=#m-1
set #d=#d + DATEDIFF(d, #dt, EOMONTH(#dt))
END
IF(#m<0)
BEGIN
Set #y=#y-1
Set #m=#m+ 12
END
RETURN (cast(#y as varchar)+ ' years, ' + cast(#m as varchar) +' months, '+ cast(#d as varchar) + ' days' )
END
go
SELECT dbo.getDateSum(dbo.getDateDiff('2010-02-01','2011-03-11'),dbo.getDateDiff('2010-02-02', '2011-03-11'))
go