Denodo is a data virtualization tool, which means Denodo is adding a layer of abstraction between consuming applications and data sources that you might be connecting to for the purpose of your use case, irrespective of the structure of data coming from individual data source.
Coming to your specific questions:
1. If you are looking for running a very basic query on an existing table with your Oracle database, I am sure there is no requirement to add any abstraction layer. But I am assuming your use cases would be more complex than that.
Importing from underlying data sources into Denodo as base views just creates a standard representation in Denodo format irrespective of the underlying data source (which could have SQL tables, web services, excels or any other type of data returned). You can combine views from same/different data sources and also create new fields with combined data without making changes to your data source. So, this process simplifies querying of data, instead of complicating it.
2. Denodo does not store any data, but queries it in real time form the data sources (unless you choose to [cache]( https://community.denodo.com/docs/html/browse/7.0/vdp/administration/cache_module/cache_module) the data). Denodo gives you an option:
* To combine multiple objects (for example tables) from the same data source during the creation of base views, or call any function that resides in the data source, when you choose to create a [base view through query]( https://community.denodo.com/kb/view/document/Using%20the%20Create%20Base%20View%20From%20Query%20Option?category=Data+Sources).
* To choose from a wide range of [functions]( https://community.denodo.com/docs/html/browse/7.0/vdp/vql/appendix/syntax_of_condition_functions/syntax_of_condition_functions) to enable you to massage the data. However, this can be done only in the derived views, because base views just provide the structure if the underlying data source. Data can be consumed by executing these views. You can also develop [Custom Functions]( https://community.denodo.com/docs/html/browse/7.0/vdp/developer/developing_extensions/developing_custom_functions/developing_custom_functions) if you want something beyond what is provided by Denodo.
Hope this helps!