Populating ListView from multiple SQLite databases + Online query

I have a web service which provides static data from different vendors. I am developing an Android App to provide access the web service's data. Also users are able to download vendor databases in order to have offline access and improved query speed.

The question is while I have multiple local SQLite databases and my online web service(to get query from Databases that are not local), what would be the most efficient way to get the query and merge the results to display in a ListView?

I do not need the source code, just the structure, components and tips on how to achieve an efficient solution.