What's the Most Efficient Way to Extract a Date from a DATETIME Value in SQL Server?

Efficiently extract dates from date and time in SQL Server

Efficiently extracting dates from combined date and time values ​​in SQL Server is critical for data manipulation tasks. In this case, the question is how to get the date from a datetime value like '2008-09-25 12:34:56'.

A variety of approaches have been proposed, including:

• CAST(FLOOR(CAST(CRETS AS FLOAT)) AS DATETIME)

This method converts the datetime value to a float, performs a floor operation to truncate the decimal part, and then converts the result back to datetime. Although very efficient, it was observed that this method is not the fastest in all situations.

• CONVERT(DATETIME, CONVERT(VARCHAR(10), CRETS, 120) , 120)

This method consists of converting the datetime value to a string of length 10 and then converting the string back to datetime if style 120 is specified (forcing date-only conversion).

• DATEADD(DAY, DATEDIFF(DAY, 0, CRETS), 0)

This method calculates the number of days between a datetime value and midnight, then adds that difference to midnight to get the date.

Performance Analysis

To determine the most efficient approach, we performed performance tests on large tables containing timestamps accurate to milliseconds. The following execution times were observed:

• Pure SELECT: 33803 ms
• Floor-cast: 33545 milliseconds
• String-convert: 33843 milliseconds
• DateAdd: 33440 milliseconds

Based on these results, DateAdd is slightly faster in certain test cases. Note that performance may vary based on data distribution, server configuration, and workload characteristics.

Therefore, when choosing the most efficient method to extract dates and times from SQL Server, it is recommended to evaluate various methods through performance testing based on specific data set and workload requirements.

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