group by excluding a column
To avoid grouping by a specific column that returns multiple values, you can either remove it from the query, or you can explicitly tell it which value you want. You can do this using aggregate or analytic functions, like:


MAX(ExtensionID)
MIN(ExtensionID)
FIRST_VALUE(ExtensionID) OVER (...)
LAST_VALUE(ExtensionID) OVER (...)


For numeric values you can also present new values, such as:


SUM(Duration)
AVG(Duration)


But basically, if you want to show a single row collapsed from two different ExtensionID values, you need to define how to do that. In this case, is ExtensionID important? If it is, do you want to show the most recent one? The first one? The one that occurs most frequently? What about ties? There are solutions to all of these, but you have to know what you want your query to return.

You can't just leave things out of the GROUP BY like you can with MySQL, where you get - let's say - weird results.