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Which method provides the best performance when removing the time portion from a datetime field in SQL Server?

In SQL Server, there are multiple ways to truncate the time component from a datetime value, but the performance differences among them are typically minor. However, some techniques can be more efficient than others and also more readable or maintainable. Below are the most common methods:

1. Casting to date (SQL Server 2008+)

SELECT CAST(SomeDateTimeColumn AS date) AS TruncatedDate
FROM YourTable;

Pros

  • Very clean, readable syntax.
  • Requires no additional function calls or string operations.
  • Directly uses the built-in date type introduced in SQL Server 2008.

Cons

  • Only available in SQL Server 2008 and later.

2. Using CONVERT(date, SomeDateTimeColumn)

SELECT CONVERT(date, SomeDateTimeColumn) AS TruncatedDate
FROM YourTable;

Pros

  • Similar benefits to CAST(... AS date).
  • Also straightforward and readable.

Cons

  • Same version requirement (SQL Server 2008+).

3. Using DATEADD and DATEDIFF

SELECT DATEADD(day, DATEDIFF(day, 0, SomeDateTimeColumn), 0) AS TruncatedDateTime
FROM YourTable;

How It Works

  1. DATEDIFF(day, 0, SomeDateTimeColumn) calculates the number of days between “day zero” (i.e., 1900-01-01) and the given datetime.
  2. DATEADD(day, <days>, 0) then adds that number of days back to 1900-01-01, resulting in a date with a zero time component (00:00:00.000).

Pros

  • Works in all SQL Server versions (including those before SQL Server 2008).
  • Efficient numeric calculation rather than string manipulation.

Cons

  • Less intuitive to read compared to casting to date.
  • Returns a datetime with time set to 00:00:00.000, not a date type (though for many use cases that’s fine).

4. Converting to a String, Then Back to datetime

SELECT CONVERT(datetime, CONVERT(varchar(10), SomeDateTimeColumn, 121)) AS TruncatedDateTime
FROM YourTable;

This approach:

  1. Converts the date to a string of length 10 (e.g., YYYY-MM-DD).
  2. Converts the truncated string back to a datetime.

Pros

  • Also supported in older SQL Server versions.

Cons

  • Involves string manipulation, which is generally slower than numeric operations.
  • Less readable (two conversions needed).
  • Potential collation or localization issues if not careful with formats.

Which Method Is Fastest?

1. CAST(... AS date) or CONVERT(date, ...)

  • Likely the best balance of simplicity, readability, and performance for SQL Server 2008+.
  • Internally, SQL Server simply extracts the date portion without additional overhead of string or numeric manipulation.

2. DATEADD/DATEDIFF

  • Historically known as the best solution before SQL Server 2008 introduced the date data type.
  • It’s still highly efficient because it’s purely numeric-based and avoids string operations.

In modern SQL Server versions (2008+), the difference in performance between CAST/CONVERT(date, ...) and the DATEADD/DATEDIFF approach is minimal. Benchmark tests often show them performing nearly on par, with CAST/CONVERT(date, ...) possibly being marginally faster or about the same in most workloads.

Key Takeaway:

  • For SQL Server 2008+, CAST(SomeDateTime AS date) (or CONVERT(date, SomeDateTime)) is generally considered the best approach—it’s clean, maintainable, and efficient.
  • For older SQL Server versions, DATEADD/DATEDIFF is the preferred numeric trick to strip time from a datetime.

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Conclusion

For removing the time portion of a datetime in SQL Server 2008 and later, CAST(SomeDateTimeColumn AS date) or CONVERT(date, SomeDateTimeColumn) is both the simplest and one of the fastest options. If you’re on an older SQL Server version, the DATEADD/DATEDIFF method remains a highly efficient alternative.

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