How to Efficiently Round Time Values to the Nearest 15-Minute Interval in T-SQL?

T-SQL: Precise Time Rounding to the Nearest 15 Minutes

Efficiently rounding time values to the nearest 15-minute increment is a common database task. This article demonstrates a streamlined approach using built-in T-SQL functions, avoiding the complexities of user-defined functions or CASE statements.

The objective is to round time values to the nearest quarter-hour. For example, "00:08:00" rounds up to "00:15:00," while "00:07:00" rounds down to "00:00:00."

A highly efficient method involves combining the DATETADD and DATEDIFF functions. The formula leverages the difference in minutes from a fixed starting point (e.g., '2000-01-01') to calculate the nearest 15-minute interval. This avoids potential DATEDIFF integer overflow issues that can occur with very large date ranges.

The formula works as follows:

1. DATEDIFF(minute, '2000-01-01', YourTimeValue): Calculates the total minutes between a fixed date and your time value.

2. / 15: Divides the total minutes by 15 to determine the number of 15-minute intervals.

3. * 15: Multiplies the result by 15 to get the nearest multiple of 15 minutes.

4. DATETADD(minute, ..., '2000-01-01'): Adds the calculated minutes back to the fixed date, yielding the rounded time value.

Therefore, the complete formula to round to the nearest 15 minutes is:

<code class="language-sql">DATETADD(minute, (DATEDIFF(minute, '2000-01-01', YourTimeValue) / 15) * 15, '2000-01-01')</code>

Replace YourTimeValue with your actual time column or variable.

This technique is adaptable to other rounding intervals. For instance, to round to the nearest 30 seconds, simply substitute second for minute and 30 for 15 in the formula. The key is using a fixed starting date to prevent overflow errors. This approach provides a concise and efficient solution for precise time rounding within T-SQL.

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