Just a caveat to add about DateDiff, it counts the number of times you pass the boundary you specify as your units, so is subject to problems if you are looking for a precise timespan.
e.g.
select datediff (m, '20100131', '20100201')
gives an answer of 1, because it crossed the boundary from January to February, so even though the span is 2 days, datediff would return a value of 1 - it crossed 1 date boundary.
Gives a value of 1, again, it passed the minute boundary once, so even though it is approx 14 seconds, it would be returned as a single minute when using Minutes as the units.
Internally in SQL Server dates are stored as 2 integers. The first integer is the number of days before or after the base date (1900/01/01). The second integer stores the number of clock ticks after midnight, each tick is 1/300 of a second.
So this isn't my answer but I just found this while searching around online for a question like this as well. This guy set up a procedure to calculate hours, minutes and seconds. The link and the code:
--Creating Function
If OBJECT_ID('UFN_HourMinuteSecond') Is Not Null
Drop Function dbo.UFN_HourMinuteSecond
Go
Exec(
'Create Function dbo.UFN_HourMinuteSecond
(
@StartDateTime DateTime,
@EndDateTime DateTime
) Returns Varchar(10)
As
Begin
Declare @Seconds Int,
@Minute Int,
@Hour Int,
@Elapsed Varchar(10)
Select @Seconds = ABS(DateDiff(SECOND ,@StartDateTime,@EndDateTime))
If @Seconds >= 60
Begin
select @Minute = @Seconds/60
select @Seconds = @Seconds%60
If @Minute >= 60
begin
select @hour = @Minute/60
select @Minute = @Minute%60
end
Else
Goto Final
End
Final:
Select @Hour = Isnull(@Hour,0), @Minute = IsNull(@Minute,0), @Seconds = IsNull(@Seconds,0)
select @Elapsed = Cast(@Hour as Varchar) + '':'' + Cast(@Minute as Varchar) + '':'' + Cast(@Seconds as Varchar)
Return (@Elapsed)
End'
)
There are a number of ways to look at a date difference, and more when comparing date/times. Here's what I use to get the difference between two dates formatted as "HH:MM:SS":
ElapsedTime AS
RIGHT('0' + CAST(DATEDIFF(S, StartDate, EndDate) / 3600 AS VARCHAR(2)), 2) + ':'
+ RIGHT('0' + CAST(DATEDIFF(S, StartDate, EndDate) % 3600 / 60 AS VARCHAR(2)), 2) + ':'
+ RIGHT('0' + CAST(DATEDIFF(S, StartDate, EndDate) % 60 AS VARCHAR(2)), 2)
I used this for a calculated column, but you could trivially rewrite it as a UDF or query calculation. Note that this logic rounds down fractional seconds; 00:00.00 to 00:00.999 is considered zero seconds, and displayed as "00:00:00".
If you anticipate that periods may be more than a few days long, this code switches to D:HH:MM:SS format when needed:
ElapsedTime AS
CASE WHEN DATEDIFF(S, StartDate, EndDate) >= 359999
THEN
CAST(DATEDIFF(S, StartDate, EndDate) / 86400 AS VARCHAR(7)) + ':'
+ RIGHT('0' + CAST(DATEDIFF(S, StartDate, EndDate) % 86400 / 3600 AS VARCHAR(2)), 2) + ':'
+ RIGHT('0' + CAST(DATEDIFF(S, StartDate, EndDate) % 3600 / 60 AS VARCHAR(2)), 2) + ':'
+ RIGHT('0' + CAST(DATEDIFF(S, StartDate, EndDate) % 60 AS VARCHAR(2)), 2)
ELSE
RIGHT('0' + CAST(DATEDIFF(S, StartDate, EndDate) / 3600 AS VARCHAR(2)), 2) + ':'
+ RIGHT('0' + CAST(DATEDIFF(S, StartDate, EndDate) % 3600 / 60 AS VARCHAR(2)), 2) + ':'
+ RIGHT('0' + CAST(DATEDIFF(S, StartDate, EndDate) % 60 AS VARCHAR(2)), 2)
END
I can mention four important functions of MS SQL Server that can be very useful:
1) The function DATEDIFF() is responsible to calculate differences between two dates, the result could be "year quarter month dayofyear day week hour minute second millisecond microsecond nanosecond", specified on the first parameter (datepart):
select datediff(day,'1997-10-07','2011-09-11')
2) You can use the function GETDATE() to get the actual time and calculate differences of some date and actual date:
select datediff(day,'1997-10-07', getdate() )
3) Another important function is DATEADD(), used to convert some value in datetime using the same datepart of the datediff, that you can add (with positive values) or substract (with negative values) to one base date:
select DATEADD(day, 45, getdate()) -- actual datetime adding 45 days
select DATEADD( s,-638, getdate()) -- actual datetime subtracting 10 minutes and 38 seconds
4) The function CONVERT() was made to format the date like you need, it is not parametric function, but you can use part of the result to format the result like you need:
select convert( char(8), getdate() , 8) -- part hh:mm:ss of actual datetime
select convert( varchar, getdate() , 112) -- yyyymmdd
select convert( char(10), getdate() , 20) -- yyyy-mm-dd limited by 10 characters
DATETIME cold be calculated in seconds and one interesting result mixing these four function is to show a formated difference um hours, minutes and seconds (hh:mm:ss) between two dates:
declare @date1 datetime, @date2 datetime
set @date1=DATEADD(s,-638,getdate())
set @date2=GETDATE()
select convert(char(8),dateadd(s,datediff(s,@date1,@date2),'1900-1-1'),8)
... the result is 00:10:38 (638s = 600s + 38s = 10 minutes and 38 seconds)
Another example:
select distinct convert(char(8),dateadd(s,datediff(s, CRDATE , GETDATE() ),'1900-1-1'),8) from sysobjects order by 1
Ok we all know the answer involves DATEDIFF(). But that gives you only half the result you may be after. What if you want to get the results in human-readable format, in terms of Minutes and Seconds between two DATETIME values?
The CONVERT(), DATEADD() and of course DATEDIFF() functions are perfect for a more easily readable result that your clients can use, instead of a number.