If you want to use the Mendix database as a data source, you can choose either ‘Database’ or ‘XPath’ as the data source type.
You cannot use an external SQL database as the data source directly, because the data coming from the external database needs to be mapped to Mendix objects before it can be displayed. Therefore, if you want to use an external DB, you will need to use a microflow combined with activities from e.g. Database Connector to fetch the data. The fetched data will be mapped to objects in the domain model you’ve set up to model the result set, which can then be displayed.
There isn’t to my knowledge a way to fetch data directly from an external database and display it in a list view without using a microflow to map the result set to mendix objects.