Database Designer for MySQL supports the following diagram functions:
Merge Diagram
Allows to merge content of two diagrams.
Check Diagram
Allows to check your database diagram for most typical errors and defects.
Compare Diagrams
Allows you to compare diagram with another diagram or physical database.
Diagram Versioning (Revisions)
Allows you to create "snapshots" of your diagram state after particular stages of diagram development. Later you can return your diagram to one of previous revisions, see changes between two revisions or even generate SQL-code for changing database structure from one diagram revision to another.
Diagram Synchronization
Allows to synchronize your diagram with changes in physical database. For example you changed several tables in your database directly using ALTER TABLE queries and you want to have this changes in your diagram.