How to Efficiently Calculate Running Totals in SQL Server?

Calculate running total in SQL Server

Introduction

Calculating running totals is a common task in data analysis. In SQL Server, there are several ways to achieve this, including using window functions, correlated subqueries, and cursor-based solutions. The Aggregate Set Statement Technique is a way to spark discussion. This technique involves inserting rows into a temporary table with an empty running total, updating the table in a specific order, and finally selecting the updated running total.

Aggregation set statement skills

The following query uses the aggregate set statement trick to calculate a running total:

INSERT INTO @AnotherTbl(id, somedate, somevalue, runningtotal) 
SELECT id, somedate, somevalue, null
FROM TestTable
ORDER BY somedate

DECLARE @RunningTotal int
SET @RunningTotal = 0

UPDATE @AnotherTbl
SET @RunningTotal = runningtotal = @RunningTotal + somevalue
FROM @AnotherTbl

This method is considered efficient because it updates rows in the order they are retrieved. However, there are concerns about its reliability in all scenarios.

An alternative to the aggregate set statement trick

1. Window function:

SQL Server 2012 introduced the OVER clause, allowing the use of window functions (such as SUM with cumulative aggregation). This eliminates the need for additional table operations and simplifies queries:

SELECT somedate, somevalue,
    SUM(somevalue) OVER(ORDER BY somedate) AS RunningTotal
FROM TestTable

2. Related subquery:

A correlated subquery can be used to retrieve the running total for each row:

SELECT somedate, somevalue,
(
    SELECT SUM(somevalue) 
    FROM TestTable b 
    WHERE b.somedate < a.somedate
) AS RunningTotal
FROM TestTable a

3. Cursor-based solution:

Cursors provide a mechanism to process rows step by step (in a specific order). The following cursor-based solution incrementally calculates the running total:

DECLARE @TotalTable table(id int primary key, somedate date, somevalue int, runningtotal int)

DECLARE forward_cursor CURSOR FAST_FORWARD 
FOR 
SELECT id, somedate, somevalue
FROM TestTable 
ORDER BY somedate


OPEN forward_cursor 

DECLARE @running_total int, 
    @id int, 
    @somedate date, 
    @somevalue int
SET @running_total = 0

FETCH NEXT FROM forward_cursor INTO @id, @somedate, @somevalue 
WHILE (@@FETCH_STATUS = 0)
BEGIN
     SET @running_total = @running_total + @somevalue
     INSERT @TotalTable VALUES(@id, @somedate, @somevalue, @running_total)
     FETCH NEXT FROM forward_cursor INTO @id, @somedate, @somevalue 
END

CLOSE forward_cursor
DEALLOCATE forward_cursor

SELECT * FROM @TotalTable

Conclusion

In SQL Server, the choice of method for calculating running totals depends on factors such as data size, performance requirements, and SQL Server version. The "aggregate set statement trick" is still a viable option for efficient updates, and window functions provide a more straightforward syntax in newer versions of SQL Server. Correlated subqueries and cursor-based solutions should be considered for flexibility and control. Benchmarking is recommended to determine the best approach for a specific scenario.

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