Entities and Attributes

The Entity–Relationship (ER) model represents real-world data using three fundamental concepts:

• Entities

• Attributes

• Relationships

This section focuses on entities and attributes, which form the building blocks of conceptual database design. Relationships are introduced later.

---

1. Entities and Attributes

What Is an Entity?

An entity is a real-world object or concept that has an independent existence. Entities can be:

• Physical objects: person, employee, car, house

• Conceptual objects: company, job, university course

Each entity is distinguishable from others and is described by a set of attributes.

---

Attributes of an Entity

An attribute is a property or characteristic that describes an entity.

Example: EMPLOYEE entity

• Name

• Age

• Address

• Salary

• Job

Each entity has one value per attribute (unless the attribute is multivalued). These attribute values form the actual data stored in the database.

---

2. Types of Attributes in the ER Model

The ER model classifies attributes into several categories based on their structure and usage.

---

2.1 Simple vs. Composite Attributes

Simple (Atomic) Attributes

• Cannot be subdivided into smaller components

• Example: Age, Salary, Sex

Composite Attributes

• Can be divided into smaller, meaningful sub-attributes

• Example:

  • Address → Street, City, State, Zip

  • Street → Number, Street_name, Apartment_number

📌 Key Point
Composite attributes are useful when users sometimes refer to the entire attribute and sometimes to its individual components.

---

2.2 Single-Valued vs. Multivalued Attributes

Single-Valued Attributes

• Have exactly one value for each entity

• Example: Age of a person

Multivalued Attributes

• Can have multiple values for the same entity

• Example:

  • Phone_numbers of a person

  • College_degrees of a student

  • Colors of a car

📌 Constraints on Multivalued Attributes

• Lower and upper bounds may restrict how many values are allowed

• Example: A car may have at most two colors

---

2.3 Stored vs. Derived Attributes

Stored Attributes

• Values are explicitly stored in the database

• Example: Birth_date, Salary

Derived Attributes

• Values are calculated from other attributes

• Example:

  • Age derived from Birth_date

  • Number_of_employees derived by counting employees in a department

📌 Design Note
Derived attributes are often not stored to avoid redundancy and inconsistency.

---

2.4 NULL Values

A NULL value represents the absence of a valid value for an attribute. There are two main meanings of NULL:

1. Not Applicable

• Attribute does not apply to the entity

• Example:

  • Apartment_number for a single-family house

  • College_degrees for a person with no degree

2. Unknown

• Attribute applies, but the value is not known

• Example:

  • Home_phone number not provided

🔍 Further Classification of Unknown NULL

• Missing value: Value exists but is not recorded

• Unknown existence: It is not known whether the value exists at all

---

2.5 Complex Attributes

Attributes can be both composite and multivalued, and these can be nested arbitrarily.

Complex Attribute

• Combination of composite and multivalued attributes

• Represented using:

  • Parentheses ( ) for composite attributes

  • Braces { } for multivalued attributes

Example: Address_phone

• A person may have multiple residences

• Each residence may have:

  • One address (composite)

  • Multiple phone numbers (multivalued)

📌 Such nested structures are called complex attributes.

{Address_phone( {Phone(Area_code,Phone_number)},Address(Street_address

(Number,Street,Apartment_number),City,State,Zip) )}

---

3. Importance of Attributes in Conceptual Design

• Attributes define what information is stored

• Proper classification helps:

  • Reduce redundancy

  • Improve clarity

  • Support correct constraints

• Understanding attribute types is essential for:

  • Designing ER diagrams

  • Mapping ER schemas to relational schemas

---

4. Key Takeaways

• Entities represent real-world objects with independent existence.

• Attributes describe the properties of entities.

• Attributes can be:

  • Simple or composite

  • Single-valued or multivalued

  • Stored or derived

• NULL values represent missing, unknown, or inapplicable data.

• Complex attributes allow flexible modeling of real-world data.