Came across this while looking for software to reverse engineer the mythtv database because the schema details on the mythtv wiki are lacking IMO.
There is an ERD entry on wikipedia at http://en.wikipedia.org/wiki/Entity-relationship_model
which references some more software.
There is a schema browser in TOra (supposed to have mysql support), but a quick install under ubuntu doesn't give me access.
A search for "tora mysql" returns a .rpm plugin, but I get a connection refused message from sourceforge if I try downloading it.