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Cardinality of Attributes

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Given an entity E and an attached attribute A:

• Optional attribute if min-card(A,E)=0; e.g., the attribute emp_address of the entity employee


• Mandatory attribute if min-card(A,E)=1; i.e., columns with NOT NULL feature such as the attribute student_name of the entity student.


• Single-valued attribute if max-card(A,E)=1; e.g., the attribute sid of the entity student


• Multi-valued attribute if max-card(A,E)=N; e.g., the attribute hobbies of the entity employee


• card(A,E) = (x,y) = (min-card(A,E),max-card(A,E))

The figure below shows the cardinality of attributes:

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I know John is bad for me, but when I get a look at his           baby blues (blue eyes) I can’t resist him.