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Even if no sales have been made, the total of the sales for the year is not unknown. It is zero. But consider another related column that is called LastSaleDate. When no sales have been made, there is no logical value that could be placed in this column. Note that database designers will argue as to whether or not a column such as LastSaleDate should even exist as it could be derived from the lack of sales but often such columns are created for pragmatic performance reasons. Another common reason for allowing a column to be NULL is because of the lack of information on what eventually will be placed in the column.

DEFAULT Constraint At times a column is mandatory, that is, a value must be provided for the column. However, the application or program that is inserting the row might not be providing a value for that column. It may wish to have a rule applied by which the value of the column is automatically generated. DEFAULT constraints are associated with a database column. They are used to provide a default value for the column when no value is supplied by the user. The value is retrieved from the evaluation of an expression and the data type returned by the expression must be compatible with the data type of the column.

14-4 Implementing a Microsoft® SQL Server® 2008 R2 Database Data Integrity Across Application Layers Key Points Data integrity can be applied at different levels within an application. There is no right and wrong answer for all situations. Application Levels Applications are often structured in levels. This is done to keep related functionality together and to improve the maintainability of code and the chance of it being reusable. Common examples of application levels are: • • • User interface level Middle tier (sometimes referred to as business logic) Data tier Data integrity could be enforced at each of these levels.

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