Modeling Multiple Valued Attributes:
We know that some attributes have multiple values. For example, the attribute
'Skill' of EMPLOYEE entity may have multiple values. It means that an employee
may have more than one skill. In E-R diagram, double line ellipse is used to
indicate such type of attributes as shown in figure below.
Diagram:
In conceptual design process, multi-valued attributes are often removed from
the entities for which they appear. Each multi-valued attribute (or each
repeating group) is converted into, a separate entity and then it is connected
to the entity type from which it was removed.
In the figure below, the new entity type SKILL with relationship to the EMPLOYEE
entity is shown. There is one-to-many relationship between EMPLOYEE and SKILL
entities. The SKILL entity has 'Skill-Name' and 'EmpNo' (used as foreign key)
attributes. The 'EmpNo' attribute has been included in both the entities to join
them. Typically, to become a successful entity, SKILL must have more than one
attribute.
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