Types of attribute in relational model

 

            Attribute Types

 

„          Each attribute of a relation has a name

 

„          The set of allowed values for each attribute is called the domain of the attribute

 

„          Attribute values are (normally) required to be atomic; that is, indivisible

 

z  E.g. the value of an attribute can be an account number, but cannot be a set of account numbers

 

„          Domain is said to be atomic if all its members are atomic

 

„          The special value null is a member of every domain

 

„          The null value causes complications in the definition of many operations

 

z  We shall ignore the effect of null values in our main presentation and consider their effect later