If you really really want to use an RDBMS to store the actual ZODB, you can search for AdaptableStorage or DirectoryStorage, but that's quite advanced stuff and you'd need a good reason to do it.
AdaptableStorage (Ape, now, right?) maybe: it can do anything. DirectoryStorage is a file-system based storage, with nothing and nothing else to do with relational databases. Unless you consider the filesystem to be an RDB, in which case you're wrong. (The filesystem is a database, and usually a pretty good one, but it is not relational.) There are various specialized RDB-based storages: BerkleyDB, Oracle, maybe others. These map the ZODB into a relational database. I don't know that I'd prefer these over FileStorage or DirectoryStorage. --jcc