Learn how to fix the “Aggregate Functions Cannot Be Nested” Error in PostgreSQL. Our PostgreSQL Support team is here to help you with your questions and concerns.

“Aggregate Functions Cannot Be Nested” Error in PostgreSQL

If you have been running into the “Aggregate functions cannot be nested” in PostgreSQL, you have come to the right place.

Interestingly, this error often occurs when we try to use an aggregate function within the arguments of another aggregate function.

Aggregate functions, like SUM, AVG, COUNT, and MAX, perform calculations on sets of values and return a single consolidated result.

Today, we are going to take a look at this error and how to overcome it.

Before we begin, let’s take a look at an example of a nested aggregate function:

SELECT AVG(SUM(column_name)) FROM table_name;

Here, the inner aggregate function, SUM(column_name), acts as an argument for the outer aggregate function, AVG. However, PostgreSQL does not allow such nesting. This is because aggregate functions operate on sets of values. So, nesting them will prevent potential ambiguity or undefined behavior.

Fortunately, we can fix the error with subqueries or common table expressions (CTEs). Hence, they will calculate the inner aggregate result before applying the outer aggregate function. Let’s explore both approaches.

Solution 1: Using Subqueries

Here is an example of how subqueries can help us resolve the aggregate nesting error in PostgreSQL.

SELECT AVG(inner_sum) FROM (
SELECT SUM(column_name) AS inner_sum FROM table_name
) AS subquery_alias;


Here, the inner query calculates the sum (SUM(column_name)) and names it as inner_sum. Then, the outer query then computes the average (AVG(inner_sum)) based on the result of the inner query.

Solution 2: Using Common Table Expressions (CTEs)

So, here is an example of how to use Common Table Expressions to fix the aggregate nesting error in PostgreSQL.

WITH cte AS (
SELECT SUM(column_name) AS inner_sum FROM table_name
)
SELECT AVG(inner_sum) FROM cte;


Here, CTE offers a cleaner and more readable structure to the query, particularly beneficial in more complex scenarios.

Let us know in the comments which one of the above solutions helped you out.

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Conclusion

Today, our Support Engineers demonstrated how to fix the “Aggregate Functions Cannot Be Nested” Error in PostgreSQL.