How Can I Efficiently Compare Only the Time Portion of DATETIME Values in SQL Server?

Comparing Time Values in SQL Server

When comparing time values stored in a datetime field, it's essential to consider whether the date component should be included in the comparison. In the scenario provided, the user aims to compare only the time part, excluding the date.

The proposed query using the CONVERT function to convert the time values to a string may work, but it can be inefficient. A more optimal solution is to exploit the internal floating-point representation of datetime values in SQL Server.

To perform a time-only comparison efficiently, we can:

1. Convert the datetime values to float using the CAST function.
2. Subtract the fractional part of each float (time component) using the MOD operator.

For example, given two datetime values @first and @second:

DECLARE @first DATETIME = '2009-04-30 19:47:16.123';
DECLARE @second DATETIME = '2009-04-10 19:47:16.123';

SELECT (
    CAST(@first AS FLOAT) - FLOOR(CAST(@first AS FLOAT))
) - (
    CAST(@second AS FLOAT) - FLOOR(CAST(@second AS FLOAT))
) AS Difference;

This approach directly compares the time values without involving the date component, providing accurate and efficient results.

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