mysql to Postgres
In recent years Many a developer has been spoilt by the ease of installing and using mysql. Thus instead of learning SQL they have learnt mySQL - Many of those who have fallen into this pitfall would happen to be PHP developers.
Fortunately for me when I first learned SQL I learned ANSI SQL and not mysql - even though most of my practising was on mysql database. That was around five years ago. Since then I have done a bit of work with M$ SQL server and worked for a couple (bad) days on an Oracle DB. Other than that it has been mysql all the way.
Now I find myself working with PostgreSQL and I am having to relearn a few things because of the bad habits aquired by working on mysql. Today I moved one of my databases from mysql to postgres and found myself spending a lot of time RTFMing.
So what made me switch from being an advocate of mysql for many years to an advocate of postgresql - a database that don't even know all that well? The short answer is mysql licensing issues.
The mysql AB intepretation of the GPL is radically different from everyone else's. GPL is bad enough as it is being the worst of the open source licenses. That's why they should never have switched the client libraries from LGPL to GPL in the first place. If mysql's intepretation of the GPL is valid, Micro$oft's outlook mail client would have to be open source too.
The reason that mysql is so popular is that developers - PHP developers in particular banded around it, answered messages in newsgroups, put up tutorials and blogged about it. Mysql ab seems to have turned their back on this community and that popularity will wane.