When you connect to a database and begin designing or modifying a database diagram or a table, the Visual Database Tools retain your work in memory. That is, the tools do not transmit your work to the database until you explicitly save the work there. Regardless of how many database diagrams you open or how many tables you design, the Visual Database Tools retain a single in-memory model of the database structure. There are several ramifications:
Because your modifications are not saved to the database immediately, you can experiment to see how a proposed modification will affect the database. When you complete you modifications, you can either save your changes to the database, save your changes to a script file, or discard your changes.
When you modify a database object, every open diagram containing that object will reflect that modification. For example, if you add a column to a table, the new column appears on every open diagram containing that table. If you modify an object and later add that object to another diagram, the added object reflects the modifications — even if you have not yet saved the modifications to the database.
The in-memory model of the database structure endures until you close all database diagrams and table design windows for that database. Thus, it is possible that the in-memory model retains modifications that are not visible on any open database diagrams. Even if you remove a modified object from the only open diagram, the modification remains in memory. If you later add the same object to any diagram, the modification will be visible.
Note Query Designer windows and View Designer windows use a different strategy to retain your work in memory. For more information, see Interactions Among Query and View Designer Windows