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

Solution 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.

Solution 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.

Solution 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.

Solution 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

Solution 5

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

View more solutions

Comments

[Portman]

Actually, that's 25% more characters than dateadd(day,datediff(day,0,@datetime),0), so it's not easier to type. It's also 15% less efficient.

[Portman]

We've found that CONVERT() can be anywhere from 10% to 5x less performant than dateadd/datediff. SQL imposes a penalty for converting between numeric types and strings and then back again.

[MatBailie]

The date you calculate your offset from doesn't need to be in the past. Any date will work, provided it is itself 'FLOOR'ed to the interval in questions. If the base date is in the future, you just get a negative offset value...

[Hogan]

@Portman - is there a basis for your claim it is 15% less efficient?

[Davin Studer]

To floor to the week use this if Sunday is the first day of the week ... select dateadd(week,datediff(week,0,@datetime),-1)

[Davin Studer]

Use this if Monday is the first day of the week ... select dateadd(week,datediff(week,0,@datetime),0)

[Rik]

casting to floor hurts performance since it will skip any datetime indexes. When using SQL 2008 it's better to use the datediff functions or CAST( [field] AS TIME) or CAST( [field] as DATE)

[Hogan]

A prior answer suggested this and was commented on (slower)

[Adam]

Not sure why this answer isn't at the top, this is a very efficient for date formatting in Microsoft SQL Server

[droid]

This doesn't work in SQL Server 2008 R2: Explicit conversion from data type date to float is not allowed. stackoverflow.com/a/5505975/1919692

[mahendra rajeshirke]

@droid - I just tested it in SQL Server 2014, and it works fine.

[N8allan]

I suggest that you use case lower(@DatePart) when 'year'..., rather than case when lower(... to avoid all the unnecessary code and lower conversions.