How Can I Avoid 'not a GROUP BY expression' Errors When Using Aggregate Functions in SQL?

Understanding SQL's GROUP BY Clause and Aggregate Functions

Using SQL's GROUP BY clause often leads to confusion about which attributes are accessible after grouping. Let's clarify:

GROUP BY and Non-Aggregated Columns

Attempting to access non-aggregated columns after grouping results in a "not a GROUP BY expression" error. This is because each group might contain multiple rows, and the values in non-aggregated columns will differ across those rows.

Aggregate Functions: A Deeper Look

Conversely, aggregate functions (like SUM, MAX, AVG, etc.) can access underlying columns within each group even after applying GROUP BY. They calculate a single summary value for the entire group.

Example:

Consider this query:

SELECT *
FROM order_details
GROUP BY order_no;

This query will fail because it tries to select all columns without using aggregate functions on non-aggregated columns such as order_price.

This, however, is a valid query:

SELECT SUM(order_price)
FROM order_details
GROUP BY order_no;

Here, SUM processes each group, summing the order_price values to give a total price for each order_no.

Including Non-Aggregated Columns in GROUP BY

To access non-aggregated columns after grouping, include them in the GROUP BY clause:

SELECT order_no, SUM(order_price)
FROM order_details
GROUP BY order_no;

This correctly retrieves each order_no along with its total order_price.

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