Mermaid Entity Relationship Diagrams

calendarDec 12, 2024 · 2 min read · mermaid erd entity-relationship database diagram diagrams  ·
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Entity Relationship Diagrams (ERD) visualize database schemas, showing entities, their attributes, and relationships. Perfect for database design and documentation.

Use Case

Use ER diagrams when you need to:

  • Design database schemas
  • Document database structure
  • Show relationships between entities
  • Visualize data models
  • Communicate database architecture

Code

1```mermaid
2erDiagram
3    CUSTOMER ||--o{ ORDER : places
4    ORDER ||--|{ LINE-ITEM : contains
5    PRODUCT ||--o{ LINE-ITEM : "ordered in"
6```

Result:

Explanation

  • erDiagram - Start ER diagram
  • Entity names in UPPERCASE
  • Relationship syntax: Entity1 ||--o{ Entity2 : label
  • Cardinality symbols:
    • ||--|| : One to one
    • }o--|| : Many to one
    • }o--o{ : Many to many
    • ||--o{ : One to many
    • }o--|{ : One or more to many

Examples

Example 1: E-Commerce Database

 1```mermaid
 2erDiagram
 3    CUSTOMER {
 4        int customer_id PK
 5        string name
 6        string email
 7        string address
 8    }
 9    
10    ORDER {
11        int order_id PK
12        int customer_id FK
13        date order_date
14        decimal total
15    }
16    
17    PRODUCT {
18        int product_id PK
19        string name
20        decimal price
21        int stock
22    }
23    
24    ORDER_ITEM {
25        int order_id FK
26        int product_id FK
27        int quantity
28        decimal price
29    }
30    
31    CUSTOMER ||--o{ ORDER : places
32    ORDER ||--|{ ORDER_ITEM : contains
33    PRODUCT ||--o{ ORDER_ITEM : "ordered in"
34```

Result:

Example 2: University Database

 1```mermaid
 2erDiagram
 3    STUDENT {
 4        int student_id PK
 5        string name
 6        string email
 7    }
 8    
 9    COURSE {
10        int course_id PK
11        string title
12        int credits
13    }
14    
15    ENROLLMENT {
16        int student_id FK
17        int course_id FK
18        string grade
19    }
20    
21    PROFESSOR {
22        int professor_id PK
23        string name
24        string department
25    }
26    
27    STUDENT }o--o{ COURSE : enrolls
28    COURSE }o--|| PROFESSOR : "taught by"
29```

Result:

Relationship Cardinality

SymbolMeaningDescription
`--
`}o--`
`--o{`
}o--o{Many to ManyMany entities relate to many
`}o--{`One or More to Many

Notes

  • Entity names should be in UPPERCASE
  • Attributes are defined inside curly braces
  • PK = Primary Key, FK = Foreign Key
  • Relationship labels are optional but recommended
  • Use descriptive relationship names

Gotchas/Warnings

  • ⚠️ Entity Names: Must be uppercase and use hyphens for multi-word names
  • ⚠️ Attributes: Define inside entity blocks with type and constraints
  • ⚠️ Relationships: Cardinality symbols must match the relationship direction
  • ⚠️ Complexity: Large ER diagrams can become hard to read - break into modules

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