Foreign key constraints are supported for the
InnoDB storage engine only. For other storage
engines, the foreign key syntax is correctly parsed but not
implemented. For more information, see
Foreign Key Differences.
Using MySQL Workbench you may add a foreign key from within the table editor or by using the relationship tools on the vertical toolbar of an EER Diagram. This section deals with adding a foreign key using the foreign key tools. To add a foreign key using the table editor, see Section 18.104.22.168, “The Foreign Keys Tab”.
The graphical tools for adding foreign keys are most effective when you are building tables from the ground up. If you have imported a database using an SQL script and need not add columns to your tables, you may find it more effective to define foreign keys using the table editor.