Calculating a Rolling Sum in SQL Server: Summing Over Defined Rows

Discover how to efficiently calculate a `rolling sum` over a defined number of rows in SQL Server without relying on dates. This guide walks through the steps with practical examples.
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Calculating a Rolling Sum in SQL Server: Summing Over Defined Rows

In the world of SQL Server, calculating aggregates like sums is a common requirement for data analysis and reporting. One interesting and often tricky task is summing a column over a defined number of rows rather than all rows. In this guide, we will break down how to accomplish a 4-week rolling sum without using date constraints, ensuring that you get the correct totals based on the last four entries.

The Problem: Summing Over Defined Rows

Imagine you have a table containing hours worked by individuals, segmented by week. For instance, you would like to sum the hours column for a specific individual over the last four weeks of data. Below is a sample structure of this table:

weekweek_numbernamehours2021-01-251John252021-02-083John102021-02-154John202021-03-087John252021-03-229John20From the data above, you want to find the total hours for the last four rows related to John (weeks 3, 4, 7, and 9), which should give you a total of 75 hours. With that in mind, let's delve into a solution that can help you achieve this objective.

The Solution: Using Window Functions

To accomplish this task, we can leverage SQL Server's window functions to compute the sums dynamically based on the specified number of rows. Here's how you can do it:

SQL Query

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Query

Selecting the Necessary Columns: We start by selecting the columns we are interested in from our table (my_table), including the week, week_number, name, and hours.

Using the SUM Window Function: The key to achieving our rolling sum is the SUM() window function:

PARTITION BY name: This clause ensures that the summing operation is done separately for each individual. In our case, this means John will have his own rolling sum calculated.

ORDER BY week: This clause dictates the order in which the rows are processed. It’s crucial for creating an accurate rolling sum based on the weeks.

ROWS BETWEEN 3 PRECEDING AND CURRENT ROW: This is the heart of our rolling calculation. It defines the window of rows for the summation:

3 PRECEDING means that we want to include the previous three rows plus the current row in the sum.

Outputting the Rolling Sum: The result of the query will include a new column called rolling_sum, which contains the total hours worked calculated over the last four rows for each week entry.

Conclusion

By using SQL Server's powerful window functions, calculating a rolling sum over a defined number of rows becomes a clearer and more efficient process. Through the steps outlined above, you can confidently sum up to four entries without the need for less intuitive methods, all while ensuring accurate and dynamic reporting. Implementing this approach will allow you to run more sophisticated queries and drive insights from your data effectively.

Now you have the tools at your disposal to conduct your 4-week rolling sum analysis in SQL Server – happy querying!