In this article Key Constraints in ER Model we give the information about A key constraint specifies how many instances of one entity can be associated with instances of another entity in a relationship.
Key Constraints in ER Model:
- Key Constraints
Definition:
A key constraint specifies how many instances of one entity can be associated with instances of another entity in a relationship.
It is used to enforce uniqueness and maintain data integrity in relationships.
Types of Key Constraints:
| Relationship Type | Description | Symbol | Example |
| One-to-One (1:1) | One entity from A is related to one from B | Arrow from both entities | One Person has one Passport |
| One-to-Many (1:N) | One entity from A is related to many from B | Arrow from the “one” side | One Teacher teaches many Students |
| Many-to-One (N:1) | Many entities from A are related to one from B | Arrow from the “many” side | Many Employees belong to one Department |
| Many-to-Many (M:N) | No uniqueness — many on both sides | No arrows | Students enroll in many Courses |
ER Diagram Notation:
- Represented by an arrow (→) from the entity side that has at most one associated entity in the relationship.
- If an entity has total participation and is on the “one” side, we use a double line and arrow.
Example (One-to-Many):
[Teacher] →——— (teaches) ——— [Student]
- One teacher teaches many students.
- The arrow indicates that each student is taught by only one teacher.
- Weak Entity Sets
Definition:
A weak entity set is an entity that cannot be uniquely identified by its own attributes alone. It relies on a related strong entity set and a discriminator (also called a partial key).
Characteristics of Weak Entities:
- Does not have a primary key
- Identified using a foreign key + partial key
- Has total participation in a relationship with the strong entity
- The relationship is called an identifying relationship
Notation in ER Diagrams:
| Component | Notation |
| Weak Entity | Double Rectangle |
| Identifying Relationship | Double Diamond |
| Total Participation | Double Line |
| Discriminator Attribute | Dashed/Partial Key (dotted underline) |
Example:
Scenario:
A Dependent (child, spouse) is a weak entity. It cannot exist without an Employee (strong entity). Each dependent has a Name, but many dependents may have the same name.
ER Diagram Structure:
[Employee] —=⟹ (has) =— [Dependent]
| |
[EmpID, Name] [Name (partial key), Age]
- Dependent is a weak entity
- has is the identifying relationship
- EmpID + Name = unique identifier for Dependent
Comparison Table: Key Constraints vs Weak Entity Sets
| Feature | Key Constraints | Weak Entity Sets |
| Purpose | Limit how many entities can be related | Handle entities without full identifiers |
| Main Concept | Cardinality (1:1, 1:N, N:M) | Existence depends on another entity |
| Symbol | Arrow on relationship | Double rectangle and double diamond |
| Uses Primary Key? | Yes | No – uses foreign key + discriminator |
| Participation Requirement | Can be total or partial | Always total in identifying relationship |
| Example | Teacher teaches Students (1:N) | Dependent of Employee |
Conclusion
- Key Constraints define how many entities can relate in a relationship (1:1, 1:N, M:N), ensuring uniqueness and consistency.
- Weak Entity Sets represent entities that depend on another for identification, useful in real-world systems like Dependents, Order Items, or Sub-accounts.
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