I am researching the topic -- counting databases by installed base is
quite a bit harder than counting value.

My current best estimate is that there are about 20 million SQL
database installations. This number excludes desktop databases (i.e.
Access and alike, about 40 million) and non-SQL embedded stuff
(hundreds of millions).

The current best estimate for Firebird is 3 million installations, or
about 15% of the total. MySQL claims 5 million, but I would think
that their real number is also around 3 million. Oracle is about a
million and so are DB2 and SQLServer. If you add in MSDE, SQLServer
reaches about 3 million too.

I would welcome all feedback and input on this topic, because I
intend to write a paper about it.

Thanks in advance!

Paul