XML to DDL is a set of Python programs that converts an XML representation of a database into a set of SQL (or DDL commands - Data Definition Language) commands.
Also, you can download the XML metadata directly from your existing database.
Other tools exist to examine the difference between two XML schemas and output a sequence of SQL statements
to change from one to the other (normally via ALTER statements).
There is also a tool to create HTML documentation from the XML.
XML to DDL supports PostgreSQL, MySQL, Oracle and Firebird databases.