SchemaSpy is a Java-based tool that analyzes the metadata of a
schema in a database and generates a visual representation of it in a
browser-displayable format. It lets you click through the hierarchy of
database tables via child and parent table relationships as represented
by both HTML links and entity-relationship diagrams. It's also designed
to help resolve the obtuse errors that a database sometimes gives related
to failures due to constraints.

SchemaSpy uses JDBC's database metadata extraction services to gather the
majority of its information, but has to make vendor-specific SQL queries
to gather some information such as the SQL associated with a view and
the details of check constraints. The differences between vendors have
been isolated to configuration files and are extremely limited. Almost
all of the vendor-specific SQL is optional.