I created a custom post type 'Book', for which data is stored in an external database (like ISBN, author etc). The WordPress database contains the ISBN per custom post, which connects the correct data to the custom post via a custom database connection.
The ISBN number is stored in the 'content' field of the custom post. The single-book template extracts the ISBN from the post content, and then loads in the additional info based on the ISBN from the external database.
In the backend, on the 'All Books' page, I want to use sortable columns (for instance, sort on author name). I know that sorting for custom post types works fine with meta values, however since the data of the custom post type is stored in a separate database, these meta values are nonexistant. Is there any way I can sort the posts using the data from the external database (i.e. by not using meta values which would contain duplicate data)?
As a side note, I'm creating this from scratch and plugins aren't the solution in this particular case.