To remove a table from a database diagram
If the table has unsaved changes as a result of edits made in the database diagram by you or another user, a message prompts you to save the table before removing it.
Note If you create a new table, then remove it from the diagram before saving it to the database; the table name remains in memory until you close the database diagram. Also, if you delete an existing table from the database, the table name remains in memory.
Removing a Table from a Database Diagram