Exam 3: The Relational Database Model

____ yields a vertical subset of a table.

To be considered minimally relational,the DBMS must support the key relational operators ____,PROJECT,and JOIN.

Referential ____ means that if the foreign key contains a value,that value refers to an existing valid tuple (row)in another relation.

Character data can contain any character or symbol intended for mathematical manipulation.

The following example exhibits ____________________ integrity: The CUSTOMER table's primary key is CUS_CODE.The CUSTOMER primary key column has no null entries,and all entries are unique.Similarly,the AGENT table's primary key is AGENT_CODE,and this primary key column also is free of null entries.

A ____ is any key that uniquely identifies each row.

The DIFFERENCE operator subtracts one table from the other.

The proper use of ____________________ keys is crucial to controlling data redundancy.

The relational operators have the property of ____________________;that is,the use of relational algebra operators on existing relations (tables)produces new relations.

____ logic,used extensively in mathematics,provides a framework in which an assertion (statement of fact)can be verified as either true or false.

A(n)____ join only returns matched records from the tables that are being joined.

There is never a good reason to use null values in a database.

In a natural join,the column on which the join was made occurs twice in the new table.

Fortunately,the problems inherent in the ____________________ relationship can easily be avoided by creating a composite entity.

You can think of a table as a persistent representation of a logical relation.

Depending on the sophistication of the application development software,nulls can create problems when functions such as COUNT,AVERAGE,and SUM are used.

A(n)____________________ key can be described as a superkey without unnecessary attributes.

A left outer join on tables CUSTOMER and AGENT yields all of the rows in the CUSTOMER table,including those that do not have a matching value in the AGENT table.

Describe the use of nulls in a database.

The ____ relational type is the "relational model ideal."