Issues of Database Evolution

If you change the structure of a deployed database, you must take special care that your alteration is compatible with the existing data and database structure. You might need to take special steps when you make the following modifications:

• Adding a Constraint   If you add a constraint, the database might already contain data that does not satisfy it. When you try to save the new constraint, the Save Incomplete dialog box informs you that the database could not create the constraint. To force the database to accept the new constraint, you can clear the Check existing data on creation check box.
• Adding a Relationship   If you add a relationship, the database might already contain rows of the foreign-key table that do not have corresponding rows in the primary-key table. That is, the existing data might not satisfy referential integrity. When you try to save the new relationship, the Save Incomplete dialog box informs you that the database could not save the revised foreign-key table. To force the database to accept the modification, you can clear the Check existing data on creation check box. 
• Modifying a Table Contributing to an Indexed View   If you modify a table that contributes to a Microsoft SQL Server indexed view, the indexes on the view will be lost. See the SQL Server documentation for information on recreating indexes.

No matter how you alter the database design, you should retain a history of the alterations.