Automatically create ER diagram from database schema
The relationship between tables in any relational database is done using Foreign Keys.

From the Wikipedia definition:


  Foreign keys play an essential role in database design. One important part of database design is making sure that relationships between real-world entities are reflected in the database by references, using foreign keys to refer from one table to another.


Without FKs there will be no way for any schema modelling tool of knowing that two tables are related.

A solution might be for you to script out FKs by parsing the names of the tables (if the related tables have similar names), then names and data types of columns that define the relationship. But that's no trivial task.