is there some standard about how to graphically represent database schemas? Is UML a defacto standard for this?
Also, is there some free tool that can help me convert an sql file (basically bunch of CREATE TABLE queries) into some nice graph (UML or not UML)?
You are looking for ERD:
http://en.wikipedia.org/wiki/Entity-relationship_model
As for a tool, visual Paradigm UML can help you model (with both UML and ERD). The community edition allows you to create ERD diagrams.
UML? No. Entity/relationship diagrams are de riguer for databases.
MySQL Workbench does a nice job.
UML is an object approach while database is a relational concept. You can't therefore model your database with UML but you can transform your code into Java then reverse engineer this code with java modeling tools. I found an example with EclipseUML at: http://www.forum-omondo.com/documentation_eclipseuml_2008/Eclipse_Database/Reverse_Existing_Database/index.html