In the Entity Relationship (ER) Model, relationship types describe how many instances of one entity can be associated with instances of another. The three main relationship types are one-to-one (1:1), one-to-many (1:M), and many-to-many (M:N). Understanding these types is essential for correct ER design and mapping to relational tables.

One-to-One (1:1) Relationship

In a one-to-one relationship, one instance of entity A is related to at most one instance of entity B, and vice versa.

Examples

  • One PERSON has at most one PASSPORT.

  • One ROOM in a hostel may be assigned to one STUDENT.

ER Notation

Usually written as 1:1 on the relationship line between two entities.

Mapping to Relational Model

  • Add a foreign key to one of the tables (usually the dependent one).

For example, in:

PERSON(PID, Name)

PASSPORT(ID, Issue_Date)

you may add PID as a foreign key in the PASSPORT table.

One-to-Many (1:M) Relationship

In a one-to-many relationship, one instance of entity A can relate to many instances of entity B, but each B instance relates to at most one A instance.

Examples

  • One DEPARTMENT can have many EMPLOYEES, but each EMPLOYEE belongs to one department.

  • One AUTHOR may write many BOOKS, but each BOOK has one author (in a simple model).

ER Notation

Written as 1:M near the relationship line (1 on the DEPARTMENT side, M on the EMPLOYEE side).

Mapping to Relational Model

  • Place the foreign key in the “many” side table.

Example

DEPARTMENT(DID, DName)

EMPLOYEE(EID, Name, DID)

The DID in EMPLOYEE is a foreign key referencing DEPARTMENT(DID).

Many-to-Many (M:N) Relationship

In a many-to-many relationship, many instances of entity A can relate to many instances of entity B.

Examples

  • Many STUDENTS enrolled in many COURSES.

  • Many CUSTOMERS may order many PRODUCTS.

ER Notation

Written as M:N between the two entities.

Mapping to Relational Model

  • Create a separate junction table (also called relationship table or associative entity).

  • This table has foreign keys from both sides plus any relationship attributes.

Example

STUDENT(Roll_No, Name, Branch)

COURSE(Course_ID, Course_Name, Credits)

ENROLLMENT(Roll_No, Course_ID, Grade, Semester)

Here, ENROLLMENT is the junction table with:

  • Roll_No (foreign key from STUDENT)

  • Course_ID (foreign key from COURSE)

  • Primary key: {Roll_No, Course_ID}

Why Relationship Types Matter

  • They determine where to place foreign keys in the relational model.

  • They affect data integrity and constraints (NOT NULL, uniqueness).

  • They influence query design (JOINs between tables).

Summary

Relationship types in the DBMS ER Model are one-to-one (1:1), one-to-many (1:M), and many-to-many (M:N). Each type has a specific meaning and leads to a particular way of mapping entities into relational tables. By correctly identifying the relationship type, beginners can design more accurate and consistent database schemas.