Database diagrams can range in complexity from just one table to hundreds of tables. When you’re working with a large number of tables, you can create a "master" diagram that contains all the tables you need, and then divide the master diagram into many smaller diagrams or "subdiagrams."
Subdivide your diagram when you want to isolate a group of the tables. For example, if you are designing an application with many modules, you can create one master diagram with all the tables that your application needs. You can then subdivide the master diagram so that each subdiagram contains only the tables pertaining to a specific module.
To subdivide a database diagram
Because each table that appears in a database diagram is merely a reference to a database table, you need to edit the table in only one diagram in order for your changes to be immediately reflected in each diagram in which the table appears.