Does Spring Data JDBC support joins between entities?
I'm afraid you misread the documentation.
Query derivation is not supported in the 1.0 version of Spring Data JDBC.It certainly will be added in the not too far future.
The misunderstanding stems from the fact that all Spring Data documentation starts with a general part outlining the features in principle available to modules. This part is the same for all modules.And then the module specific part which describes the actual features. Unfortunately, the fact that Query Derivation is not supported can only be deducted from the fact it isn't mentioned in the module specific part.
Once this feature arrives it most likely will support querying across entities, but at least in the beginning only across entities of the same aggregate.
The concept of aggregates is extremely important for Spring Data JDBC, which is why there is a blog article about this concept and its ramifications for Spring Data JDBC, which I highly recommend for reading.