Why Does 'SELECT MAX(SUM(TIME)) FROM downloads GROUP BY SSN' Produce a 'Not a Single-Group Group Function' Error?

Error: "Not a Single-Group Group Function" Explained

When executing the SQL statement, "SELECT MAX(SUM(TIME)) FROM downloads GROUP BY SSN", you encounter an error stating "not a single-group group function." This error arises because the maximum expression, MAX(SUM(TIME)), is a group function that operates on the sum of time for each Social Security Number (SSN) in the downloads table. However, the additional inclusion of SSN in the SELECT statement creates a conflict.

To understand why this conflict occurs, consider the following explanation:

• The SUM(TIME) operation calculates the total downloads for each SSN, resulting in a single value for each SSN.
• The MAX function then operates on these single values, returning the maximum sum of downloads among all SSNs.

The presence of the SSN column in the SELECT list violates the "single-group group function" rule. This rule requires that all columns included in the SELECT statement must be part of the GROUP BY clause. In this case, SSN is not part of the GROUP BY clause, making the query invalid.

To resolve this issue, you can choose one of the following actions:

• Remove SSN from the SELECT statement: This will return only the maximum sum of downloads without identifying the associated SSN.
• Add SSN to the GROUP BY clause: This will group the results by both SSN and the sum of downloads, allowing you to include SSN in the SELECT list.

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