Floor a date in SQL server - 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.

Answer 2

In SQL Server here's a little trick to do that:
SELECT CAST(FLOOR(CAST(CURRENT_TIMESTAMP AS float)) AS DATETIME)
You cast the DateTime into a float, which represents the Date as the integer portion and the Time as the fraction of a day that's passed. Chop off that decimal portion, then cast that back to a DateTime, and you've got midnight at the beginning of that day.
This is probably more efficient than all the DATEADD and DATEDIFF stuff. It's certainly way easier to type.

Answer 3

Expanding upon the Convert/Cast solution, in Microsoft SQL Server 2008 you can do the following:
cast(cast(getdate() as date) as datetime)
Just replace getdate() with any column which is a datetime.
There are no strings involved in this conversion.
This is ok for ad-hoc queries or updates, but for key joins or heavily used processing it may be better to handle the conversion within the processing or redefine the tables to have appropriate keys and data.
In 2005, you can use the messier floor: cast(floor(cast(getdate() as float)) as datetime)
I don't think that uses string conversion either, but I can't speak to comparing actual efficiency versus armchair estimates.

Answer 4

I've used #Portman's answer many times over the years as a reference when flooring dates and have moved its working into a function which you may find useful.
I make no claims to its performance and merely provide it as a tool for the user.
I ask that, if you do decide to upvote this answer, please also upvote #Portman's answer, as my code is a derivative of his.
IF OBJECT_ID('fn_FloorDate') IS NOT NULL DROP FUNCTION fn_FloorDate
SET ANSI_NULLS OFF
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE FUNCTION [dbo].[fn_FloorDate] (
#Date DATETIME = NULL,
#DatePart VARCHAR(6) = 'day'
)
RETURNS DATETIME
AS
BEGIN
IF (#Date IS NULL)
SET #Date = GETDATE();
RETURN
CASE
WHEN LOWER(#DatePart) = 'year' THEN DATEADD(YEAR, DATEDIFF(YEAR, 0, #Date), 0)
WHEN LOWER(#DatePart) = 'month' THEN DATEADD(MONTH, DATEDIFF(MONTH, 0, #Date), 0)
WHEN LOWER(#DatePart) = 'day' THEN DATEADD(DAY, DATEDIFF(DAY, 0, #Date), 0)
WHEN LOWER(#DatePart) = 'hour' THEN DATEADD(HOUR, DATEDIFF(HOUR, 0, #Date), 0)
WHEN LOWER(#DatePart) = 'minute' THEN DATEADD(MINUTE, DATEDIFF(MINUTE, 0, #Date), 0)
WHEN LOWER(#DatePart) = 'second' THEN DATEADD(SECOND, DATEDIFF(SECOND, '2000-01-01', #Date), '2000-01-01')
ELSE DATEADD(DAY, DATEDIFF(DAY, 0, #Date), 0)
END;
END
Usage:
DECLARE #date DATETIME;
SET #date = '2008-09-17 12:56:53.430';
SELECT
#date AS [Now],--2008-09-17 12:56:53.430
dbo.fn_FloorDate(#date, 'year') AS [Year],--2008-01-01 00:00:00.000
dbo.fn_FloorDate(default, default) AS [NoParams],--2013-11-05 00:00:00.000
dbo.fn_FloorDate(#date, default) AS [ShouldBeDay],--2008-09-17 00:00:00.000
dbo.fn_FloorDate(#date, 'month') AS [Month],--2008-09-01 00:00:00.000
dbo.fn_FloorDate(#date, 'day') AS [Day],--2008-09-17 00:00:00.000
dbo.fn_FloorDate(#date, 'hour') AS [Hour],--2008-09-17 12:00:00.000
dbo.fn_FloorDate(#date, 'minute') AS [Minute],--2008-09-17 12:56:00.000
dbo.fn_FloorDate(#date, 'second') AS [Second];--2008-09-17 12:56:53.000

Answer 5

The CONVERT() function can do this as well, depending on what style you use.

Answer 6

Too bad it's not Oracle, or else you could use trunc() or to_char().
But I had similar issues with SQL Server and used the CONVERT() and DateDiff() methods, as referenced here

Answer 7

There are several ways to skin this cat =)
select convert(datetime,convert(varchar,CURRENT_TIMESTAMP,101))

Answer 8

DateAdd along with DateDiff can help to do many different tasks. For example, you can find last day of any month as well can find last day of previous or next month.
----Last Day of Previous Month
SELECT DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE()),0))
LastDay_PreviousMonth
----Last Day of Current Month
SELECT DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE())+1,0))
LastDay_CurrentMonth
----Last Day of Next Month
SELECT DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE())+2,0))
LastDay_NextMonth
Source

Answer 9

Since PostgreSQL is also a "SQL Server", I'll mention
date_trunc()
Which does exactly what you're asking gracefully.
For example:
select date_trunc('hour',current_timestamp);
date_trunc
------------------------
2009-02-18 07:00:00-08
(1 row)