In an ER Diagram, a double ellipse is used to represent : 

The correct answer is Multi-valued Attribute.

Key Points

• A Multi-valued Attribute in an Entity-Relationship (ER) diagram is an attribute that can hold multiple values for a single entity.
  • In an ER diagram, a multi-valued attribute is represented by a double ellipse.
  • For example, a person entity might have a multi-valued attribute "Phone Numbers" because a person can have more than one phone number.
  • Multi-valued attributes are used to represent situations where an entity can have multiple instances of the same attribute.
  • These attributes are essential for accurately modeling real-world scenarios where entities can possess multiple values for a particular attribute.
  • Proper representation of multi-valued attributes helps in designing a normalized database schema.

Additional Information

• In database design, it's often necessary to create a separate table to handle multi-valued attributes to maintain normalization.
• Entity-Relationship diagrams are a vital tool for database design as they help visualize the relationships between different entities.
• Understanding how to represent different types of attributes in an ER diagram is crucial for designing efficient and accurate databases.
• Other attribute types in ER diagrams include simple attributes (represented by a single ellipse) and composite attributes (represented by an ellipse containing other ellipses).