Found this Fermilab summary where a comparison of these 3 popular relational database management systems (RDBMS) was made back in 2005.
Part of the points that were extracted from the summary:
According to CD Times magazine dated July 1, 2004, Top Deployed Databases poll shows following databases in use:
SQL Server – 78%
Oracle – 55%
MySQL – 33%
PostgreSQL – 8%.
That’s interesting. I always thought that Oracle is the market leader in RDBMS used by corporates and enterprises?
Specific market segments occupied
MySQL: Web servers. Common on Linux-based systems.
PostgreSQL: Education, Telephony, DNS Services, Web services.
There’s also an article here about MySQL versus PostgreSQL.
* All these references were written back in 2005 and might be a bit outdated.