How to Efficiently Remove Time from DateTime Fields in SQL Server?

SQL Server: Efficiently Removing Time from DateTime Data

Working with datetime fields in SQL Server often requires isolating the date component, discarding the time. This article compares two common techniques: DATEADD/DATEDIFF and casting to char, analyzing their performance characteristics.

Method Comparison

The DATEADD/DATEDIFF approach:

a) SELECT DATEADD(dd, DATEDIFF(dd, 0, GETDATE()), 0)

This cleverly uses DATEDIFF to calculate the difference in days from the epoch (1900-01-01) and then DATEADD to reconstruct the date without the time component.

Alternatively, casting to char:

b) SELECT CAST(CONVERT(CHAR(11), GETDATE(), 113) AS DATETIME)

This converts the datetime to a character string (showing only the date) and then back to datetime.

Performance Analysis

Testing reveals that method (a) – DATEADD/DATEDIFF – generally consumes fewer CPU resources, especially with large datasets. However, it's susceptible to potential issues related to language and date formatting inconsistencies.

Method (b) – casting – might transfer slightly more data due to the intermediate char conversion, but this overhead is typically less impactful than the potential performance gains of (a).

Further Considerations

Since SQL Server 2008, a more direct method is available: casting directly to the DATE data type.

Recommendations

Both methods achieve the desired outcome, but the optimal choice depends on your application's context. For superior performance, especially with large datasets, DATEADD/DATEDIFF (method a) is usually preferred. If indexing the datetime column is critical, consider casting to DATE or ensuring the time component doesn't affect index usage.

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