Depends on the nature of the DBA: is it a production position, or a development position? I've never been or known anyone who would (or even could) do both. The vetting is different. The work is different. For production, firefighting is what matters, and your particular fires, at that (the deep bowels your chosen engine are what matter). For development, it's much more about data structure, data control, and organizational commitment to relational principles. Lots of "database" shops use the RDBMS merely to hold flat files, while others take the time to actually design an integrated schema. The former let the client coders run the show, while the latter have the database developers do so. So, three different kinds of DBA, none compatible with the others.