Quoting Benjamin Walkenhorst (krylon@gmx.net):
...
> So far I'm using the machine as an NFS-server, internet-gateway, 
> firewall. 

It's not a firewall if it's running an NFS serve and SQL server.
It's a gateway.  That's been configured a bit.

> Now I would like to add an SQL-database-server. It's not like I really 
> need it, so it won't be too bad if it does not work out. On the other 
> hand, I want to become a professional network-administrator, so I 
> should know a little more about databases. 
> Hence I would like to set up an SQL-server on my NetBSD-machine.

So jump into pkgsrc and build mysql or Postgres.  It will take you
a couple minutes and time to go get yourself a snack.

I've run them both on BSD for years (and back to SunOS 4).

> What I need to know is:
> - - Some information on databases in general and SQL specifically
It's all over the place and there are courses and book galore on it.
You can spend a career in this question if you want.

> - - Some information on existing servers. Which one do I want? I 
One for which you have resources - people who can help you or it
runs the tools you want.  I prefer Postgres, but I ran MySQL early when
it was a tad basic and icky.  Coin toss if you're just starting.

> understand there's basically two large projects, MySQL and PostgreSQL. 
> Is one of these to be preferred to the other one? I don't care for 
> top-performance, since there's only going to one client (my desktop 
> machine).

Much might depend on your goals.  The licenses differ, etc.  If you have
a tool in mind that's written for MySQL, you'll suffer a bit using Postgresql.
And vice versa.

> - - I also need to know how to configure the server, create databases and 
> so on. I guess, this is covered by the database's documentation, 
> though... 
Um, yeah.  And a billion tutorials on line.

> - - Anything else I should know? 
How much you don't know.  So put one in, get started with it, dump
the data often.  In 6 months, reevaluate your decision.  You'll be
miles further along.