Although we're technically not storing it on a database per se, what we do right now is store the process definition in a document management system ( Alfresco in our case). The DMS allows us the capability of versioning of the bpmn files. When a process is instantiated, we call a service that pulls the definition with a particular version.
The DMS also allows users who edit these definitions a facility for collaboration.
IF had to store it in a DB, I would probably store it in BLOB field with another field indicating the version of the process.