Symbols Used in E-R Model

Introduction

The Entity-Relationship (E-R) Model is a high-level conceptual data model used to describe the structure of a database. It was proposed by Peter Chen in 1976. The E-R model uses a set of standard graphical symbols to represent data and the relationships among data. An E-R diagram (ERD) is a diagrammatic representation using these symbols.

Standard Symbols Used in E-R Model

1. Rectangle ( Entity )

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

• A rectangle represents an Entity or Entity Set.
• An entity is a real-world object or thing that has an independent existence and can be distinctly identified.
• Example: STUDENT, EMPLOYEE, DEPARTMENT, COURSE

2. Double Rectangle ( Weak Entity )

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

• A double rectangle represents a Weak Entity Set.
• A weak entity is one that cannot be uniquely identified by its own attributes alone; it depends on a strong (owner) entity for its existence.
• Example: DEPENDENT (depends on EMPLOYEE), ORDER-ITEM (depends on ORDER)

3. Ellipse / Oval ( Attribute )

      (  Name  )

• An ellipse (oval) represents an Attribute.
• An attribute is a property or characteristic that describes an entity.
• Example: Name, Age, Salary, Address

4. Double Ellipse ( Multivalued Attribute )

    ((  Phone  ))

• A double ellipse represents a Multivalued Attribute.
• A multivalued attribute can hold more than one value for a single entity instance.
• Example: Phone_Number (a person can have multiple phone numbers), Skills

5. Dashed Ellipse ( Derived Attribute )

    (- - Age - -)

• A dashed (dotted) ellipse represents a Derived Attribute.
• A derived attribute is one whose value can be computed or derived from other attributes.
• Example: Age (derived from Date_of_Birth), Total_Salary (derived from Basic + Allowances)

6. Ellipse with Underlined Text ( Key Attribute )

    ( <u>Roll_No</u> )

• An ellipse with the attribute name underlined represents a Key Attribute (Primary Key).
• A key attribute uniquely identifies each entity in an entity set.
• Example: Roll_No for STUDENT, Emp_ID for EMPLOYEE

7. Dashed Underline ( Partial Key )

    ( - - Dep_Name - - )

• An attribute with a dashed underline represents a Partial Key (Discriminator).
• It is used for weak entities; it can uniquely identify weak entities only when combined with the key of the owner entity.
• Example: Dependent_Name in DEPENDENT entity

8. Diamond ( Relationship )

        < ENROLLS >

• A diamond shape represents a Relationship Set.
• A relationship describes how two or more entities are associated with each other.
• Example: ENROLLS (between STUDENT and COURSE), WORKS_FOR (between EMPLOYEE and DEPARTMENT)

9. Double Diamond ( Identifying Relationship )

       << HAS >>

• A double diamond represents an Identifying Relationship.
• It connects a weak entity to its owner (strong) entity.
• Example: HAS (between EMPLOYEE and DEPENDENT)

10. Lines ( Links )

• Lines are used to connect:
  • Entities to their attributes
  • Entities to relationships
• They show the participation and association between components.

11. Double Line ( Total Participation )

STUDENT ======= < ENROLLS >

• A double line between an entity and a relationship indicates Total Participation.
• Every entity in the entity set must participate in at least one relationship instance.
• Example: Every EMPLOYEE must work for a DEPARTMENT.

12. Single Line ( Partial Participation )

STUDENT ------- < ENROLLS >

• A single line indicates Partial Participation.
• Not every entity needs to participate in the relationship.
• Example: Not every STUDENT may enroll in every COURSE.

13. Cardinality Notations on Lines

Cardinality ratios are written on the lines to indicate the number of entity instances participating in a relationship:

• Example: One DEPARTMENT has many EMPLOYEEs (1:N)

Summary Table

Conclusion

The E-R model symbols provide a standardized, visual way to design and communicate database structure. Each symbol has a precise meaning, and together they allow database designers to represent entities, their attributes, and the relationships among them clearly before actual implementation in a DBMS.