FUNDAMENTALS OF COMPUTER

Detailed explanation-1: -6. Which one of the following cannot be taken as a primary key? Explanation: Street is the only attribute which can occur more than once.

Detailed explanation-2: -Primary keys must contain unique values. A primary key column cannot have NULL values. A table can have only one primary key, which may consist of single or multiple fields.

Detailed explanation-3: -Street addresses or driver license numbers are examples of primary keys used to uniquely identify (respectively) locations or cars. As another example, a database must hold all of the data stored by a commercial bank.

Detailed explanation-4: -To qualify as a primary key for an entity, an attribute must have the following properties: It must have a non-null value for each instance of the entity. The value must be unique for each instance of an entity. The values must not change or become null during the life of each entity instance.