How to Resolve the 'Not a Single-Group Group Function' Error in SQL?

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

When attempting to combine group functions with individual column expressions in a SELECT statement, as in the provided query:

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

You may encounter the "not a single-group group function" error. This occurs because the SUM(TIME) group function calculates a single value for each group (SSN), making it incompatible with the individual column expression SSN.

To resolve this error, you have several options:

1. Remove the Individual Column Expression:

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

   This will only return the maximum sum value without the corresponding SSN.

2. Add a GROUP BY Clause for All Column Expressions:

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

   This will group the results by both SSN and TIME, making the MAX(SUM(TIME)) function valid.

3. Use an Aggregate Subquery:

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

   This subquery calculates the maximum sum value and selects only the rows where the SUM(TIME) for each SSN matches that maximum.

Depending on your specific requirements, one of these solutions will allow you to obtain the desired results without encountering the "not a single-group group function" error.

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