Why Does My SQL Query Produce a 'Not a Single-Group Group Function' Error?

Understanding the "Not a Single-Group Group Function" Error in SQL

In SQL, a single-group group function, such as MAX(), SUM(), or AVG(), is applied to a group of rows that have the same values in the specified grouping columns. However, if you attempt to include individual column expressions in the SELECT list along with a group function and do not include all of those expressions in the GROUP BY clause, you will encounter the "not a single-group group function" error.

Explanation of the Problem

In the example SQL statement you provided:

SELECT SSN, MAX(SUM(TIME))
FROM downloads
GROUP BY SSN

the query attempts to calculate the maximum value of the sum of TIME for each unique SSN and then return the SSN associated with that maximum value. However, there is a fundamental issue with this query:

• The expression MAX(SUM(TIME)) is a single-group group function that calculates the maximum sum of TIME for each SSN. By definition, this means that there is only one result for each SSN.
• Including the individual column expression SSN in the SELECT list violates the rule that all individual column expressions included in the SELECT list must also be included in the GROUP BY clause.

Resolution

To resolve this error, you have three options:

• Drop the individual column expression (SSN) from the SELECT list: This will give you the maximum sum of TIME without the associated SSN.
• Add the individual column expression (SSN) to the GROUP BY clause: This will group the results by both SSN and TIME, allowing you to retrieve the SSN associated with the maximum sum of TIME.
• Use a subquery: You can use a subquery to retrieve the maximum sum of TIME and then use that value in a separate query to identify the associated SSN.

Example Query using Option 2

SELECT SSN, MAX(SUM(TIME))
FROM downloads
GROUP BY SSN, TIME

This query will group the results by both SSN and TIME, allowing you to retrieve the SSN associated with the maximum sum of TIME for each SSN.

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