How to Efficiently Compare Only Time Values in SQL Server?

Comparing Time in SQL Server

When comparing time values in SQL Server, it's crucial to consider only the time component and disregard the date. To achieve this, a slightly different approach is required compared to the standard comparison between datetime fields.

To efficiently compare time values, one can leverage the fact that SQL Server stores dates as floating-point numbers. The digits before the decimal represent the date, while those after it represent the time.

Example:

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

In this example, the date parts of both @first and @second are discarded by subtracting the integer part of each floating-point representation using floor(). The result is a fractional value representing the time difference, which can then be used for comparison.

Explanation:

• cast(@first as float) - floor(cast(@first as float)) calculates the time part of @first in floating-point format.
• cast(@second as float) - floor(cast(@second as float)) does the same for @second.
• Subtracting the time parts gives the difference between the two times.

This method is more efficient than converting the datetime values to strings for comparison, as it avoids unnecessary data conversion.

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