ER Diagram adds all related tables to selected table. How to add non-related tables to ER Diagram?
Use other windows in SQL Nav that list table names, such as DB Navigator, Find Objects or Code Editor. Select View | DB Navigator or View | Code Editor or Search | Find Objects menu options to open one of these windows. You can arrange these windows side-by-side with ER Diagram and then drag and drop table names to ER Diagram.
