Of course you can store other types of data in a relational database. For example you can serialise your domain state to a SQL Server database and retrieve that state at a later date but this is a terrible use of a relational system. So bad, in fact, that whole layers of code (so called 'data-layers', or DALs), many thousands of lines, are required to make this kind of task remotely do-able, testable, maintainable.
The great, decades long confidence trick has been for the SQL based retailers to convince us that there was no better way to store hierarchical / document data other than real-time conversion between radically different data-structures. DALs and the latest ORM have all been trumpeted but, in the end, these are nothing more than codecs to thunk your bits back and forth between different data organisations - different patterns on the disk.