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The product of the database design process whichaims to identify and organize the required data logically andphysically.A data model says what information is to be contained in adatabase, how the information will be used, and how the itemsin the database will be related to each other.For example, a data model might specify that a customer isrepresented by a customer name and credit card number and aproduct as a product code and price, and that there is aone-to-many relation between a customer and a product.It can be difficult to change a database layout once code hasbeen written and data inserted. A well thought-out data modelreduces the need for such changes. Data modelling enhancesapplication maintainability and future systems may re-useparts of existing models, which should lower developmentcosts.A data modelling language is a mathematical formalism with anotation for describing data structures and a set ofoperations used to manipulate and validate that data.One of the most widely used methods for developing data modelsis the entity-relationship model. The relational model isthe most widely used type of data model. Another example isNIAM.["Principles of Database and Knowledge-Base Systems",J.D. Ullman, Volume I, Computer Science Press, 1988, p. 32].