When working with dates and times in SQL Server, there are often situations where you need to round a datetime value to a specific precision, such as a day, hour, minute, or second. However, determining the most efficient way to perform this rounding can be a challenge.

In a recent conversation with a fellow SQL Server expert, Jeremiah Peschka, I discovered that there are multiple methods for rounding dates in TSQL, but it wasn’t clear which method was the most efficient. To find out, I decided to run some tests and compare the performance of each method.

Rounding to the Day

One of the most common scenarios is rounding a datetime value to the day level. For example, instead of ‘1/4/2011 6:15:03.393921’, you may want to have just ‘1/4/2011’. Fortunately, SQL Server 2008 introduced the date type, which makes this task much easier. You can simply cast a datetime or datetime2 value as a date, and you will get the desired result. Plus, the new value is stored in just 3 bytes, making it more efficient.

SELECT CAST('1/1/2010 23:59:59.000' AS DATE) AS [I'm a date!]

Rounding to Hours, Minutes, or Seconds

However, rounding to other precisions, such as hours, minutes, or seconds, can be more complicated. SQL Server’s datatypes have different opinions about what rounding means. For example, the SMALLDATETIME datatype rounds up for minutes and date values, while the DATE datatype does not. This can lead to confusion and unexpected results.

When deciding whether to use SMALLDATETIME, it’s important to understand whether you want to round up or round down for minutes and date values. Most of the time, we want to round down and get the largest minute number that is less than or equal to the datetime value. However, SMALLDATETIME does not provide this behavior, so it should be used with caution.

Comparing Methods of Rounding Dates

Now, let’s compare the efficiency of different methods for rounding dates. To do this, we will create a table with a large number of datetime values and measure SQL Server’s performance when working with these dates.

-- Create a table with datetime values
CREATE TABLE dbo.Dates (DateValue DATETIME2(7))

-- Insert a large number of datetime values
DECLARE @startDate DATETIME2(7) = '2010-01-01 00:00:00',
        @endDate DATETIME2(7) = '2010-01-31 23:59:59'

;WITH DatesCTE AS (
    SELECT @startDate AS [Date]
    UNION ALL
    SELECT DATEADD(SECOND, 1, [Date])
    FROM DatesCTE
    WHERE [Date] < @endDate
)
INSERT INTO dbo.Dates
SELECT [Date]
FROM DatesCTE
OPTION (MAXRECURSION 0)

-- Perform rounding using different methods
SELECT CAST(CONVERT(CHAR(10), DateValue, 120) AS DATE) AS [RoundedDate1],
       DATEADD(DAY, DATEDIFF(DAY, 0, DateValue), 0) AS [RoundedDate2],
       CAST(CONVERT(CHAR(10), DateValue, 120) AS DATETIME2(0)) AS [RoundedDate3]
FROM dbo.Dates

After running these queries, we can compare the CPU time and performance of each method. In my tests, I found that using the DATEADD function to calculate the number of minutes since a given date and then adding them back was the most efficient method. This is because performing mathematical functions on integer values is faster than converting between character-based datatypes.

It’s worth noting that the internal storage format of the datetime2 datatype is not explicitly documented in SQL Server’s Books Online. However, based on the performance results, it appears that datetime2 enjoys mathematical operations, suggesting that it may be stored as two four-byte integers.

In conclusion, when rounding dates in SQL Server, it’s important to consider the precision you need and choose the most efficient method for your specific scenario. By understanding the behavior of different datatypes and comparing their performance, you can optimize your queries and improve the overall efficiency of your SQL Server database.