Why is there no PRODUCT aggregate function in SQL?

sql

sql-server

custom-aggregates

workarounds

byAnton Shumikhin·Sep 16, 2024

In SQL, there's no built-in PRODUCT aggregate function due to the high risk of overflow errors as products can become exponentially large. To bypass this, we can apply logarithmic transformations:

SELECT EXP(SUM(LOG(value))) AS product
FROM numbers
WHERE value > 0;

Key Points: This solution strictly applies to positive numbers as LOG(0) and LOG(negative number) will result in a math error. Catering for datasets that include non-positive values is necessary.

Zero and Negative Values Handling

Encompassing datasets with zero or negative values, we need to add conditional logic because math doesn't like negativity (me neither, positivity rules!)

SELECT CASE
       WHEN MIN(value) = 0 THEN 0
       ELSE  --We're still friends, right, zero?
       WHEN (SUM(CASE WHEN value < 0 THEN 1 ELSE 0 END) % 2) = 0 
           THEN EXP(SUM(LOG(ABS(NULLIF(value, 0)))))
       ELSE -EXP(SUM(LOG(ABS(NULLIF(value, 0)))))
       END AS product
FROM numbers;

Note: We first check if a zero exists, and then evaluate the count of negative numbers.

Logarithmic Transforms' Drawbacks

Relying on logarithmic transformation isn't without drawbacks. Due to numerical instability arising from floating-point precision, accuracy of calculations can take a hit. After all, it's not log-ical to be perfect every time.

Custom Aggregation Functions (SQL Server)

If you're using SQL Server, you can create custom aggregates to compute products directly. SQL Server be like: "Go ahead, build-it-yourself!"

CREATE AGGREGATE Product(@value float)
RETURNS float
EXTERNAL NAME [YourAssembly].[YourNamespace].AggregateProduct;

Advanced Workarounds, Coz' Why Not?

Advanced techniques to emulate PRODUCT calculation in SQL include:

Window functions: They offer running product calculations, but it's more like a window of complexity.
T-SQL string manipulation: Concatenating string representations of numbers and executing a dynamic SQL multiply operation. A brain teaser indeed!
Custom aggregate functions: Especially in Oracle or PostgreSQL, they let you create your own flavor of complex aggregates.

Practical Scenarios to Apply the Workarounds

Mastering SQL calls for experimentation with formulas and adjusting them based on your SQL variant. Here's an opportunity to tweak solutions to fit your unique data problems: