How to Solve the 'not a single-group group function' Error in SQL?

Understanding SQL Grouping Functions: Resolving the "not a single-group group function" Error

In SQL, group functions such as SUM() aggregate values across multiple rows, grouping them by a specific column. However, when attempting to combine a group function with an individual column expression in the SELECT statement, the "not a single-group group function" error arises. This error occurs when the group function operates on a subset of the data in the same group as the individual column expression.

To resolve this issue, consider the following options:

1. Drop the Group Function:

Removing the SUM() function invalidates the aggregated value, leaving only the individual column expression.

2. Drop the Individual Column Expression:

Eliminating the SSN column from the SELECT statement will resolve the error by returning the maximum summed value.

3. Add a Comprehensive GROUP BY Clause:

Including all individual column expressions in the GROUP BY clause ensures that each row belongs to a distinct group, matching the scope of the group function. However, this option may not be suitable if you intend to group by multiple columns and have unique values for each combination.

Alternative Query for Retrieval:

For the given purpose of finding the SSN of the customer with the maximum downloads, an alternative query is recommended:

SELECT TOP 1 SSN, SUM(TIME) AS TotalTime
FROM downloads
GROUP BY SSN
ORDER BY TotalTime DESC;

This query retrieves the SSN and the total download time for each SSN, sorted in descending order based on total time. The TOP 1 clause limits the result to the SSN with the highest total time.

Alternatively, if multiple customers share the same maximum total time, the following query can be used:

SELECT
SSN, SUM(TIME) AS TotalTime
FROM downloads
GROUP BY SSN
HAVING SUM(TIME) = (SELECT MAX(SUM(TIME)) FROM downloads GROUP BY SSN);

This revised query retrieves all SSNs tied for the maximum total download time, ensuring that no information is lost.

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