Perhaps you’ve inherited a database from a previous admin? Perhaps you have some database-backed software and you need a better understanding of the schema? You’re in luck! There’s a free Java-based tool called schemaSpy. You can grab schemaSpy from sourceforge. As of this writing the latest is 4.1.1. I’ll wait while you fetch it.
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Great! Now that you have done so, please also grab the MySQL JDBC connector. You can find that here. As of this writing the latest is 5.1.7. Grab that, too.
I used that earlier today, pointing it at the database for Documenting the American South, but I’m not at liberty to share those pretty results. Instead let’s point schemaSpy at this very blog. First let’s make a directory for the output:
mkdir ~/blogschema
Now we can run schemaSpy and have it put some pretty diagrams and html there. That would look like this:
java -jar schemaSpy_4.1.1.jar -t mysql \
-dp mysql-connector-java-5.1.7/mysql-connector-java-5.1.7-bin.jar \
-hq -o ~/blogschema -host localhost -db WORDPRESSDB \
-u WORDPRESSUSER -p DBPASSWORD
Note that the stuff in caps would match what’s in the wp-config.php for the WordPress instance. Let’s unpack the rest of that a bit. The -t mysql tells schemaSpy we’re looking at a MySQL database, and will fail if you don’t have the matching -dp pointing to the .jar of the connector I had you download. The -hq means “high quality” output from graphviz (which schemaSpy uses to produce the diagrams). You really don’t want the default. Trust me; it’s ugly. After that we specify the output directory, the host that’s running the MySQL database, and the login info. NOTA BENE: you should NOT run this with the password on the command line like this on a shared system where you don’t trust the users or programs, since the password could be seen in the output of ps.
But what if (like me) your database is remote? SSH to the rescue! Open two terminal windows, and in the first, issue the following:
ssh -T -L 3306:localhost:3306 youruser@mysql.example.com
Where yourusername is your username and mysql.example.com is the host running the MySQL database. What if your login shell and the database are on different hosts? That would look like this instead:
ssh -T -L 3306:mysql.example.com:3306 youruser@login.example.com
The -T suppresses tty allocation and the -L says that the next part is the tunnel. The first 3306 means that the local port (local to you) entering the tunnel is 3306, and the second 3306 means that the remote port (the one connecting to mysql.example.com from login.example.com) of the tunnel is also 3306. Great! now this means that we can run the java -jar schemaSpy_4.1.1.jar blob as above on our local host in the other window you opened. Now let’s see what the results look like. Have a look here for that. It would be much more fun to show off the docsouth schema since it has foreign keys that make for pretty diagrams. Hopefully you have such a database you can diagram yourself. 
Happy schemaSpy-ing! Leave a comment if anything is problematic or unclear.
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