Here is a clear, neat, exam-friendly explanation of ERD symbols for:

✔ Entity
✔ Types of Entities
✔ Weak Entity
✔ Strong Entity
✔ Composite Entity (Associative Entity)

I’ll include diagrams in ASCII format so you understand the shapes.

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ERD SYMBOLS (ENTITY TYPES)

1. ENTITY (Basic Entity)

An entity is represented by a rectangle.

+-----------+
|  Entity   |
+-----------+

Example:

+-----------+
| STUDENT   |
+-----------+

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2. STRONG ENTITY (Regular Entity)

A strong entity has its own primary key and does NOT depend on another entity.

Symbol: Single rectangle.

+-----------------+
|  STUDENT        |   <-- Strong Entity
+-----------------+

Primary key is underlined:

+-------------------------+
| STUDENT                 |
| ----------------------- |
| RollNo (PK)             |
| Name                    |
| Age                     |
+-------------------------+

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3. WEAK ENTITY

A weak entity does NOT have a primary key of its own.
It depends on a strong entity for identification.

Symbol: Double rectangle

+=================+
|| DEPENDENT     ||
+=================+

Weak entity uses partial key, which is a dashed underline:

+=========================+
|| DEPENDENT             ||
|| --------------------- ||
|| Name (partial key)    ||
|| Age                   ||
+=========================+

Relationship with its strong entity is an identifying relationship (double diamond):

+-----------+     <>==<>     +=================+
| EMPLOYEE  | ----||---------|| DEPENDENT     ||
+-----------+     <>==<>     +=================+

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4. COMPOSITE ENTITY (ASSOCIATIVE ENTITY / BRIDGE ENTITY)

Used to represent M:N relationships as a separate entity.

Symbol:
A normal rectangle used between two entities, but it represents a relationship turned into an entity.

Example: Student enrolls in Course (M:N relationship)

+----------+         +-------------+         +----------+
| STUDENT  |---------| ENROLLMENT |---------| COURSE   |
+----------+         +-------------+         +----------+

Where ENROLLMENT is the composite (associative) entity.

Attributes of Composite Entity:

+-----------------------+
| ENROLLMENT            |
| --------------------- |
| RollNo (FK)           |
| CourseID (FK)         |
| DateOfJoin            |
+-----------------------+

Use case:

• Converts M:N relationship into two 1:M relationships.
• Contains attributes of the relationship (e.g., date, marks).

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5. OTHER ATTRIBUTE SYMBOLS (BONUS)

These commonly appear in exams, so included for clarity:

Simple Attribute

Single oval:

    (Name)
      |
+----------+
| STUDENT  |
+----------+

Composite Attribute

Oval with smaller ovals:

        (Address)
         /   |   \
   (City) (State) (Pincode)

Multi-valued Attribute

Double oval:

((Phone))
    |
+----------+
| STUDENT  |
+----------+

Derived Attribute

Dotted oval:

(...Age...)
      |
+-----------+
| STUDENT   |
+-----------+

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SUMMARY TABLE

Concept	Symbol
Strong Entity	Single Rectangle
Weak Entity	Double Rectangle
Composite Entity	Rectangle used for M:N relationship with FK attributes
Simple Attribute	Single Oval
Composite Attribute	Oval connected to small ovals
Multi-valued Attribute	Double Oval
Derived Attribute	Dotted Oval

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