DBMS Lecture: Entity Relationship Model (ER Model)

Introduction

• The ER model describes data as:
  • Entities
  • Attributes
  • Relationships (to be covered in future lectures)

Entities

• Defined as things with an independent existence in the real world.
• Physical Entities: House, Person, Employee
• Conceptual Entities: Course, Job

Attributes

• Properties that describe entities.
• Example:
  • For a "Person" entity, attributes might include Name, Age, Address, Phone Number.

Types of Attributes

1. Composite Attributes

   • Can be divided into parts.
   • Example: Name (First Name, Middle Name, Last Name)

2. Simple Attributes

   • Cannot be divided further.
   • Examples: Weight, Salary, Age

3. Single-Valued Attributes

   • Have one value for a particular entity.
   • Example: Age of a person

4. Multi-Valued Attributes

   • Can have multiple values for a particular entity.
   • Examples: College Degrees, Languages Known

5. Derived Attributes

   • Can be derived from other attributes.
   • Example: Age (derived from Date of Birth)

6. Stored Attributes

   • From which values of other attributes are derived.
   • Example: Date of Birth

7. Complex Attributes

   • Contain both multi-valued and composite components.
   • Example: College Degrees (multi-valued), components within parentheses (composite attributes)

Null Values

• Used when an attribute value is not applicable or unknown.
  • Not Applicable: E.g., College Degree for someone without one.
  • Unknown: E.g., Phone number of Jeremy if unknown.

Categories of Null

1. Value Exists but Missing: E.g., Joshua's age exists but is missing.
2. Existence Unknown: E.g., Unknown if Jeremy has a phone number.

Entity Type

• A collection of entities with the same attributes.
• Example: Student (entities with age attribute)

Entity Set

• A collection of entities of a specific type at a point in time.
• Example: Students aged between 19 and 23.

Key Attribute

• Identifies an entity uniquely.
• Example: Roll number or Student ID for students.

Value Set of Attributes

• Specifies the possible set of values for an attribute.
• Example: Age of an employee (integers between 22 and 60).

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These terminologies form the foundation of understanding the ER model in a DBMS context.