tsql - Floor a date in SQL server

Question

In SQL Server, how do I "floor" a DATETIME to the second/minute/hour/day/year?

Let's say that I have a date of 2008-09-17 12:56:53.430, then the output of flooring should be:

• Year: 2008-01-01 00:00:00.000
• Month: 2008-09-01 00:00:00.000
• Day: 2008-09-17 00:00:00.000
• Hour: 2008-09-17 12:00:00.000
• Minute: 2008-09-17 12:56:00.000
• Second: 2008-09-17 12:56:53.000

Answer 1

The key is to use DATEADD and DATEDIFF along with the appropriate SQL timespan enumeration.

declare @datetime datetime;
set @datetime = getdate();
select @datetime;
select dateadd(year,datediff(year,0,@datetime),0);
select dateadd(month,datediff(month,0,@datetime),0);
select dateadd(day,datediff(day,0,@datetime),0);
select dateadd(hour,datediff(hour,0,@datetime),0);
select dateadd(minute,datediff(minute,0,@datetime),0);
select dateadd(second,datediff(second,'2000-01-01',@datetime),'2000-01-01');
select dateadd(week,datediff(week,0,@datetime),-1); --Beginning of week is Sunday
select dateadd(week,datediff(week,0,@datetime),0); --Beginning of week is Monday

Note that when you are flooring by the second, you will often get an arithmetic overflow if you use 0. So pick a known value that is guaranteed to be lower than the datetime you are attempting to floor.