Why Does My SQL Query Return a 'not a single-group group function' Error?

SQL Function "not a single-group group function" Explained

In SQL, when using a group function such as MAX, the function must be applied to a single group of data. This means that if you want to find the maximum value for a specific column, you must also group the data by that column.

However, if you try to apply a group function to an expression that is not grouped, you will receive the error "not a single-group group function." For example, let's say you have a table called downloads that contains a column for downloads by a customer:

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

This query is valid because the MAX function is applied to a group of data (downloads by SSN). However, if you try to add the SSN column to the select statement, like this:

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

You will receive the "not a single-group group function" error because the SSN column is not included in the GROUP BY clause. This means that SQL cannot determine which group of data the SSN column belongs to, and therefore cannot apply the MAX function to it.

To resolve this error, you have three options:

• Drop the MAX function from the select statement, which will give you the sum of downloads for each SSN.
• Drop the SSN column from the select statement, which will give you the maximum total time downloaded by any customer.
• Add the SSN column to the GROUP BY clause, like this:

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

This will give you the maximum total time downloaded for each unique SSN and time combination.

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