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For example, it would not be sensible for us to compare a staff number with a branch number, even though the domain definitions for both these attributes are character strings. Unfortunately, you’ll find that most RDBMSs don’t currently support domains. Tuple A record of a relation. The fundamental elements of a relation are the tuples or records in the table. In the Staff relation, each record contains five values, one for each attribute. As with attributes, tuples can appear in any order and the relation will still be the same relation, and therefore convey the same meaning.

As a result, performance is generally very good. However, a DBMS is written to be more general, to cater for many applications rather than just one. The effect is that some applications may not run as fast using a DBMS as they did before. ■ Higher impact of a failure The centralization of resources increases the vulnerability of the system. Since all users and applications rely on the availability of the DBMS, the failure of any component can bring operations to a complete halt until the failure is repaired.

Views can be set up to exclude data that some users should not see. For example, we could create a view that allows a branch manager and the Payroll Department to see all staff data, including salary details. However, we could create a second view that excludes salary details, which other staff use. The database approach showing Sales and Stock Control Departments accessing the database through application programs and the DBMS. 10 ■ Background ■ Views provide a mechanism to customize the appearance of the database.

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