7.1 What is an Attribute?

Attributes are the specific properties or characteristics that describe an entity. For example, a STUDENT entity might have attributes like Name, Roll_No, Address, and Phone.

7.2 Classification of Attributes

1. Simple vs. Composite Attributes

• Simple Attributes: Cannot be further subdivided. (e.g., Gender, Age).
• Composite Attributes: Can be divided into smaller sub-parts, which represent more basic attributes with independent meanings.
• Example: Name can be split into {First_Name, Middle_Name, Last_Name}.
• Usage: Helps users refer to the whole attribute or just a component.

2. Single-valued vs. Multivalued Attributes

• Single-valued: Have a single value for a particular entity. (e.g., SSN, Birth_Date).
• Multivalued: An entity may have a set of values for the attribute.
• Example: College_Degrees (A person can have BA, MA, and PhD).
• Notation: Double Ellipse.

3. Stored vs. Derived Attributes

• Stored Attributes: Physically stored in the database. (e.g., Date_of_Birth).
• Derived Attributes: Not stored; calculated on-the-fly from other attributes.
• Example: Age (Derived from CurrentDate - Date_of_Birth).
• Notation: Dashed Ellipse.

4. Complex Attributes

Formed by nesting composite and multivalued attributes. For example, a person might have multiple addresses, and each address is composite (Street, City, Zip).

7.3 Attribute Domains

Every attribute is associated with a Domain (or Value Set), which defines the set of legal values that can be assigned to it.

• Example: The domain for Age might be integers between 0 and 120.
• Example: The domain for GPA might be real numbers between 0.0 and 4.0.