SapDB is a very capable, mature (stems from abbadas,
you dont remember that? Thats mature software for you
;-)) "enterprise" RDBMS. It actually is used in many
enteprise systems. It has seen a lot of commercial
development and was GPL`ed some years ago.
SapDB offers an oracle compatibility mode. This is good
enough for Sap apps (erp, acounting, crm) to be run
from oracle frequently.
To many people it can mean a GPL databases with a
smooth migration to oracle if need be. Perhaps, with
not much more work, it is a move the other way around
for those looking for savings on expensive oracle licences.
I would think adding Mysql and (my favourite)
postgresql takes time from other work (like adding
postgresql features). But sapdb could be a populair
choice for many of the oracle+TOra users. ODBC drivers
are avaialble, they are kind of the default.