1. Cardinality (Mapping Constraints)

Cardinality defines how many instances of one entity can be associated with instances of another entity in a relationship. It reflects quantitative constraints.

Common Cardinalities:

One-to-One (1:1)
One entity instance in A is related to at most one in B, and vice versa.
Example: Each Person has one Passport.

One-to-Many (1:N)
One entity in A relates to many in B, but each B relates to one A.
Example: One Teacher teaches many Students.

Many-to-One (N:1)
Many entities in A relate to one in B.
Example: Many Employees work in one Department.

Many-to-Many (M:N)
Many entities in A relate to many in B.
Example: Students enroll in many Courses, and each Course has many Students.

Notation:

Typically shown near the relationship line in ER diagrams: 1, N, M, etc.

2. Participation Constraints

Participation specifies whether all instances of an entity are involved in a relationship or not.

Types:

Total Participation
Every instance must participate in the relationship.
Notation: A double line connects the entity to the relationship.
Example: Every Employee must belong to a Department.

Partial Participation
Some instances may or may not participate.
Notation: A single line is used.
Example: Not all Students have LibraryAccounts.

Why it matters:

Participation tells us about mandatory or optional relationships, helping define real-world rules accurately.

3. Generalization

Generalization is a bottom-up approach where two or more similar entities are combined into a higher-level superclass based on shared characteristics.

Key Points:

• Common features go into the superclass (Vehicle).
• Specific features remain in subclasses (Car, Bike).
• Helps in data abstraction and reducing redundancy.

4. Specialization

Specialization is a top-down approach, where a broad entity is split into specialized sub-entities based on some distinct property.

Use case:

When subclasses have attributes or behavior that doesn’t apply to all members of the superclass. It helps create more accurate models.

5. Aggregation

Aggregation is used when a relationship itself needs to act as an entity. This is needed when we want to model a relationship between a relationship and another entity.

Notation:

• A dashed rectangle is drawn around the relationship and connected to another entity.

Why it’s used:

It helps model complex situations where simple relationships are not enough—especially when relationships have attributes or participate in other relationships.

Summary Table: