Chapter 2 DATABASE DESIGN FOR SQL SERVER 2000
Chapter 2 DATABASE DESIGN FOR SQL SERVER 2000 67 When attributes are decomposed, different methods of ensuring data integrity can be applied to the broken-down segments rather than the attribute as a whole. For example, a Zip attribute can be checked as an integer and City as a string. Secondly, decomposing aids in sorting and improving data retrieval performance. A generic attribute Address contains the street, city, state, and zip. To just get the state of a customer in Washington, you have to select the whole Address attribute and parse it to find Washington, thereby degrading performance because of the redundant data retrieved. If you just have four separate attributes, you can select the State column and get your results more quickly. This same rule applies to updating data. It s easier to update a single part of Address rather than parsing and then updating the whole Address attribute. In most cases, an attribute is identified for each entity that will act as a key. This attribute could be a customer number, product number, or simply an attribute to serve no other purpose but to act as a key identifier. When an attribute is added solely for the sake of being an identifier, it is known as a surrogate key. Whether you use a natural or surrogate key, that key will represent an important part in establishing relationships. The term Primary Key is assigned to this attribute. Even though SQL Server does not mandate the use of a Primary Key, one should be defined simply as a matter of good practice. General Recap on Data Modeling When designing a data model, you do not need to know how databases are actually implemented in SQL Server. In fact, the primary goal at this stage is picking the facts out of a given problem definition. A case scenario in whatever form it takes will describe the current situation, problems, and changes a firm is currently undergoing. You need to deduce all tables, columns, and relationships from this preparation. Entities become the tables when a logical design is transferred to a physical database model. Each entity is made up of a number of properties or attributes that will later become the column definitions of the table. A relationship between entities creates a logical link between the entities and in the physical design will represent the Primary Key and Foreign Key relationship of a constraint. REVIEW BREAK
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