ER Model

ER Model Symbols

Entity                → Rectangle
Relationship          → Diamond
Attribute             → Oval
Key Attribute         → Underlined Oval

Weak Entity           → Double Rectangle
Identifying Relation  → Double Diamond
Partial Key           → Dashed Underline

Total Participation   → Double Line
Partial Participation → Single Line

Multivalued Attribute → Double Oval
Derived Attribute     → Dashed Oval
Composite Attribute   → Oval with sub-ovals

Cardinality           → 1:1, 1:N, M:N (on relationship lines)

More:

ISA (is-a)            → Triangle
Specialization        → Top-down ISA
Generalization        → Bottom-up ISA
Disjoint              → ‘d’ near ISA
Overlapping           → ‘o’ near ISA

Exam Tip: If you see double boundary / double line, think dependency or total participation.

Strong Entity vs Weak Entity

Strong Entity Set

• Definition: Can be uniquely identified by its own attributes.
• Key: Has a primary key.
• Existence: Independent of other entities.
• ER Representation:
  • Strong entity → Single rectangle
  • Key attribute → Underlined
• Participation: Can be partial or total.

Example:

• Entity: Department
• Primary Key: Dept_ID
• Department exists independently of other entities.
• Participation in relationships may be partial or total.

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Weak Entity Set

• Definition: Cannot be uniquely identified by its own attributes; depends on a strong entity.
• Key: Has a partial key (discriminator).
• Existence: Dependent on strong entity.
• ER Representation:
  • Weak entity → Double rectangle
  • Identifying relationship → Double diamond
  • Partial key → Dashed underline
• Participation: Total participation is mandatory.

Example:

• Entities: Department (strong), Employee (weak)
• Relationship: WorksIn (identifying)
• Employee identified by (Dept_ID + Emp_No)
• Every Employee must be linked to a Department.

Key Difference (Exam Ready)

Feature	Strong Entity	Weak Entity
Primary Key	Present	Not present
Partial Key	Not required	Required
Existence	Independent	Dependent
Identification	By own attributes	By strong entity + partial key
Participation	Partial / Total	Always Total
ER Symbol	Single rectangle	Double rectangle

GATE Tip:
If total participation + no primary key, it is a weak entity.

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Types of Entity (ER Model)

1. Strong Entity

• Has its own primary key.
• Exists independently.

+-----------+
| Student   |
+-----------+
| Roll_No  |
| Name     |
+-----------+

2. Weak Entity

• No primary key of its own.
• Depends on a strong entity.
• Has partial key.

+-----------+      ⟪⟪ WorksIn ⟫⟫      ||-----------||
| Department|========================|| Employee  ||
+-----------+                         ||-----------||
| Dept_ID   |                         || Emp_No    ||
+-----------+                         ||-----------||

3. Associative (Intersection) Entity

• Created from M:N relationship.
• Has attributes of relationship.

+-----------+      Enrolls      +-----------+
| Student   |------------------| Course    |
+-----------+                  +-----------+

            ↓

+------------------+
| Enrollment       |
+------------------+
| Roll_No, CID     |
| Grade            |
+------------------+

4. Regular Entity

• Same as strong entity.
• Simple, independent entity.

+-----------+
| Book      |
+-----------+
| Book_ID   |
| Title     |
+-----------+

5. Subtype / Supertype Entity

• Inheritance (IS-A) relationship.
• Subtype inherits attributes.

        Employee
           |
        ---------
        |       |
     Teacher   Clerk

6. Recursive Entity

• Entity related to itself.

Employee ── Manages ── Employee

7. Dependent Entity

• Existence depends on another entity.
• Often same as weak entity.