TL;DR: AGPL database. Permissively licensed client libraries. Dual licensing the database. But really you should read the post, for the thought process that led to that approach.
Among well known database vendors, the natural comparisons are MongoDB and MySQL.
Like Mongo, RavenDB is available under a single network-copyleft license. Like Mongo, RavenDB client libraries are permissively licensed. However, MongoDB focuses on selling all-encompassing packages that include a mix of support, services, and add-ons. To hear this post tell it, Hibernating Rhinos is running more of a pure dual-licensing play with RavenDB.
That approach puts them close to MySQL. However, MySQL dual licensed both database and client libraries. Nobody needs to buy a commercial license for RavenDB client libraries.
The obvious question, in current context, is whether Hibernating Rhinos is cruising for a cloud provider bruising, the kind of which Mongo has loudly complained. Cue Server Side Public License.
Here follow shameless but relevant plugs:
Plain language, more general take on an SSPL-style license: https://github.com/kemitchell/shared-component-license/blob/...
Open form paid license agreement for indie developers dual licensing, or doing paid add-ons: https://github.com/indieopensource/paid-license
License Zero, which offers dual-licensing back-office as a service, as well as public licenses without the problems and known vulnerabilities of *GPL: https://licensezero.com/