Constraints on Binary Relationship Types

In real-world applications, not all combinations of entities are allowed in a relationship. The ER model captures these real-world rules using constraints on relationships, which are derived from the miniworld semantics.

For binary relationships, the ER model defines two fundamental types of constraints:

1. Cardinality Ratio (Maximum participation)

2. Participation Constraint (Minimum participation)

Together, these are known as structural constraints.

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1. Cardinality Ratios

The cardinality ratio specifies the maximum number of relationship instances in which an entity can participate.

Common Cardinality Ratios

Cardinality	Meaning
1:1	One entity in A is related to at most one entity in B, and vice versa
1:N	One entity in A can be related to many entities in B
N:1	Many entities in A can be related to one entity in B
M:N	Many entities in A can be related to many entities in B

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Examples from the COMPANY Database

WORKS_FOR (1:N)

• A department can employ many employees

• An employee works for at most one department

Interpretation:
DEPARTMENT : EMPLOYEE = 1 : N

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MANAGES (1:1)

• A department has at most one manager

• An employee can manage at most one department

Interpretation:
EMPLOYEE ↔ DEPARTMENT = 1 : 1

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WORKS_ON (M:N)

• An employee can work on multiple projects

• A project can have multiple employees

Interpretation:
EMPLOYEE ↔ PROJECT = M : N

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ER Diagram Notation

• 1, M, or N are written near the relationship diamond

• N indicates no upper limit

• Some notations allow exact limits (e.g., maximum of 5)

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2. Participation Constraints (Minimum Cardinality)

Participation constraints specify the minimum number of times an entity must participate in a relationship.

Types of Participation

Type	Meaning
Total participation	Every entity must participate
Partial participation	Some entities may not participate

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Examples

Total Participation (Existence Dependency)

If company policy states:

Every employee must work for a department

Then:

• EMPLOYEE has total participation in WORKS_FOR

• An EMPLOYEE entity cannot exist without being related to a DEPARTMENT

This is also called existence dependency.

ER notation:
➡️ Double line between entity and relationship

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Partial Participation

In the MANAGES relationship:

• Not every employee is a manager

• Some employees do not participate in MANAGES

Therefore:

• EMPLOYEE has partial participation

ER notation:
➡️ Single line between entity and relationship

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Structural Constraints

The combination of:

• Cardinality ratio (maximum)

• Participation constraint (minimum)

defines the structural constraints of a relationship.

These constraints precisely describe how entities interact in the miniworld.

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Visual Summary

Relationship	Cardinality	Participation
WORKS_FOR	1:N	Total for EMPLOYEE
MANAGES	1:1	Partial for EMPLOYEE
WORKS_ON	M:N	Partial for both

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Key Takeaways

✔ Cardinality ratio controls maximum participation
✔ Participation constraint controls minimum participation
✔ Total participation implies existence dependency
✔ Structural constraints ensure semantic correctness
✔ ER diagrams visually encode these rules clearly

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Teaching Tip

Students often confuse 1:N vs total participation. Emphasize:

• 1:N = how many

• Total/partial = whether participation is mandatory