Answers to Review Questions

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Answers to Review Questions

1. What is the difference between a database and a table?

A table, a logical structure that represents an entity set, is only one of the components of a database. The database is a structure that houses one or more tables and metadata. The metadata are data about data. Metadata include the data (attribute) characteristics and the relationships between the entity sets.

2. What does a database expert mean when (s)he observes that a database displays both entity integrity and referential integrity?

Entity integrity describes a condition in which all tuples within a table are uniquely identified by their primary key. The unique value requirement prohibits a null primary key value, because nulls are not unique.

Referential integrity describes a condition in which a foreign key value has a match in the corresponding table or in which the foreign key value is null. The null foreign key value makes it possible not to have a corresponding value, but the matching requirement on values that are not null makes it impossible to have an invalid value.

3. Why are entity integrity and referential integrity important in a database?

Entity integrity is important, because it means that a proper search for an existing tuple (row) will always be successful. And the failure to find a match on a row search will always mean that the row for which the search is conducted does not exist in that table. Referential integrity is important, because its existence ensures that it will be impossible to assign a non-existing foreign key value to a table. For example, the referential integrity enforcement in a SALESREP is assigned to CUSTOMER relationship means that it will be possible for a customer not have a sales rep assigned to him or her, but it will be impossible to assign non-existing sales rep to a customer.

4. A database manual notes that…...

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