How do I read the relationship symbols?

||--o{ reads "one to zero or more": one on the left, zero to many on the right. }o--|| is the reverse. Two solid dashes mean a mandatory relationship; .. means optional.

Do I have to use real database types?

You can use concrete types such as varchar and datetime or generic ones such as string and int; Mermaid does not validate type names.

My course requires Chen notation with diamond relationships

Mermaid ER diagrams use crow's-foot notation: boxes for entities and lines for relationships. If Chen notation is required, draw it with a flowchart: entities [ ], attributes (( )), relationships { }.

Course Enrolment System ER Diagram Template

Previewed in the “Indigo” theme; you can switch themes in the editor

When to use it

Database course assignments and the database design chapter of a course-registration or academic-management capstone project. Students and courses are many-to-many, resolved through the enrolment entity into two one-to-many relationships, the classic ER exam question.

Making it yours

Fields live inside the braces as type name PK/FK; the key markers are optional.
Add an entity with a new NAME { … } block and connect it with a relationship line.
Cardinality symbols: || exactly one, o{ zero or more, |{ one or more, o| zero or one; each end describes its own side.
The word on the relationship line is a verb describing what the left entity does to the right one.

Mermaid code

course-selection-er.mmd

erDiagram
    STUDENT ||--o{ ENROLMENT : enrols
    COURSE ||--o{ ENROLMENT : "is taken in"
    INSTRUCTOR ||--o{ COURSE : teaches
    DEPARTMENT ||--o{ STUDENT : administers
    DEPARTMENT ||--o{ INSTRUCTOR : employs
    STUDENT {
        string student_id PK
        string name
        string major
        int year
    }
    COURSE {
        string course_id PK
        string title
        int credits
        string instructor_id FK
    }
    ENROLMENT {
        string student_id FK
        string course_id FK
        float grade
        string term
    }
    INSTRUCTOR {
        string instructor_id PK
        string name
        string rank
    }
    DEPARTMENT {
        string department_id PK
        string name
    }

Ask an AI for one with this structure

Send the prompt below to ChatGPT or Claude, then paste the whole reply into the editor; the code inside is detected automatically.

prompt.txt

Draw a course enrolment system ER diagram in Mermaid erDiagram syntax with entities STUDENT, COURSE, ENROLMENT, INSTRUCTOR and DEPARTMENT. Students and courses are many-to-many through ENROLMENT (STUDENT ||--o{ ENROLMENT, COURSE ||--o{ ENROLMENT); an instructor teaches many courses; a department administers many students and employs many instructors. List the main fields of each entity with PK and FK markers. Output only the mermaid code block.

FAQ

How do I read the relationship symbols?: ||--o{ reads "one to zero or more": one on the left, zero to many on the right. }o--|| is the reverse. Two solid dashes mean a mandatory relationship; .. means optional.
Do I have to use real database types?: You can use concrete types such as varchar and datetime or generic ones such as string and int; Mermaid does not validate type names.
My course requires Chen notation with diamond relationships: Mermaid ER diagrams use crow's-foot notation: boxes for entities and lines for relationships. If Chen notation is required, draw it with a flowchart: entities [ ], attributes (( )), relationships { }.

More in this category

All templates

Updated 2026-09-05