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Schema modification
Dealing with new constraints (or with removal of constraints):
- Complete redesign: E-R schema design and translation to tables new schema may differ a lot from the old one; lot of work adapting interfaces
- Small changes to the schema of tables need to verify the new design; little work on the interfaces
Notes:
Sometimes a company or organization changes how it deals with information. Business rules may change. A constraint like “for every course there is only one lecturer” may be dropped by the university. A constraint like “every bank account has only one owner” may be introduced.
One approach to deal with such changes is to redo the E-R design, and then translate the new design to a structure of relations or tables. Unfortunately the new relational schema may differ a lot from the old one. This implies that a lot of the software for manipulating the information, like queries on the tables and user interfaces for entering or retrieving information may need to be redesigned and reimplemented.
A different approach is to take the existing relational schema and check whether the changes in the constraints can be handled by a small change in the table structures. Changing very little implies that the programming and the interfaces also change very little. We will therefore investigate how we can adapt relational database schemas to changes in the constraints, without reconsidering the original E-R design.