📖 What is this tool?
An entity-relationship diagram (ERD) shows database tables, their columns and how tables relate to each other through foreign keys. This generator uses Mermaid ERD syntax: you define entities with typed attributes and connect them with relationship lines (one-to-one, one-to-many, many-to-many). Choose from Blog DB, E-Commerce and SaaS templates, or model your own schema from scratch. Use it to plan a new database, document an existing one, or communicate schema changes with your team. Everything runs in your browser — your schema is never uploaded.
💡 Example
Input
erDiagram
USER ||--o{ ORDER : places
ORDER ||--|{ ITEM : contains
Output
Live ERD with entities and relationship connectors
🛠️ Common Use Cases
- Design database schemas visually
- Document table relationships for teams
- Plan data models before coding
- Review existing database structure
- Create database documentation
📝 Mermaid ERD Syntax Reference
erDiagram — starts an ERD blockUSERS { int id PK } — entity with a column (type, name, constraint)- Column constraints:
PK (primary key), FK (foreign key), UK (unique)
USERS ||--o{ ORDERS : places — one user places many orders- Cardinality symbols:
|| (exactly one), o| (zero or one), }| (one or more), }o (zero or more)
- The label after the colon describes the relationship verb (e.g. "places", "contains", "belongs to")
⚠️ Common Mistakes
- Confusing cardinality direction — read the symbols at each end of the line:
||--o{ means "exactly one to zero-or-many"
- Forgetting the relationship label — the colon and verb after the entity names are required (
USERS ||--o{ ORDERS : places)
- Using spaces in entity names — multi-word names like
Order Items break the syntax; use ORDER_ITEMS or OrderItems
- Skipping column types — while types are optional in Mermaid, including them (
int, varchar, datetime) makes the ERD useful as real documentation
❓ Frequently Asked Questions
What do the relationship symbols mean? ▾
||--|| is one-to-one, ||--o{ is one-to-many, }o--o{ is many-to-many. The | means exactly one, o means zero or one, { means one or many, } means zero or many.
How do I add attributes to entities? ▾
Inside the entity block: USER { int id PK\n string name\n string email UK }. PK = Primary Key, UK = Unique Key, FK = Foreign Key.
Can ERDs be used for NoSQL databases? ▾
ERDs are primarily for relational databases. For NoSQL, use them to show document relationships conceptually, though the strict relational semantics don't apply.
How do I show a many-to-many relationship? ▾
Use }o--o{ between the two entities. In practice, many-to-many relationships usually need a junction table — add a third entity (e.g. ENROLLMENTS) with one-to-many relationships to both sides.
Can I model inheritance or subtypes? ▾
Mermaid ERD does not support inheritance notation directly. The common workaround is to model subtypes as separate entities with a foreign key back to the parent entity.
How do I represent nullable foreign keys? ▾
Use the "zero or one" cardinality: o|--o{ means the relationship is optional on the left side. This visually indicates that the foreign key column allows NULL.
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