Entity Relationship Model

Student

Courses

One to One Mapping

erDiagram
    Student {
        int student_id PK
        string name
        date join_date
    }
    
    Studentdetails {
        int student_id PK
        string SSN
        date DOB
    }
    
    Student ||--|| Studentdetails : "1 to 1"

Student

Studentdetails

For every row on the left-hand side, there will be only one matching entry on the right-hand side.

For student id 101, you will find one SSN and one DOB.

One to Many Mapping

erDiagram
    Student {
        int student_id PK
        string name
        date join_date
    }
    
    Address {
        int address_id PK
        int student_id FK
        string address
        string address_type
    }
    
    Student ||--o{ Address : "has"

Student

Address

What do you notice here?

Every row on the left-hand side has one or more rows on the right-hand side.

For student id 101, you will notice the home address and Dorm address.

Many to Many Mapping

erDiagram
    Student {
        int student_id PK
        string name
        date join_date
    }
    
    Course {
        string course_id PK
        string course_name
    }
    
    StudentCourses {
        int student_id FK
        string course_id FK
    }
    
    Student ||--o{ StudentCourses : enrolls
    Course ||--o{ StudentCourses : offered_in

Student

Student Courses

Courses

What do you notice here?

Students can take more than one course, and courses can have more than one student.