Relational database data explorer / visualization?

Yes, i would advice you to look into DbSchema, it's a neet database management tool that will help you.

Ok, well I liked this idea so much that I've written it.

It's not released yet, but when it is it will be free.

Edit

Ok, it's now released. Free relational database exploring goodness @ http://www.atlantis-interactive.co.uk/products/datasurf/default.aspx

Edit

Although initially free, this is now part of Pragmatic Works' DBA xPress package.

DBeauty is a powerful data browser (similar to Matt Whitfield's excellent DataSurf but more powerful). It is Java based, so you need to download the JDBC driver. I've found this tool invaluable in quickly navigating data (I fell in love with Microsoft's Quadrant before they killed it off and have been looking for a replacement ever since).

Old but good and free DB subsetting tool Jailer should be able to answer the question. http://jailer.sourceforge.net/