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:
Market share
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.
Oracle: N/A
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* All these references were written back in 2005 and might be a bit outdated.