Convert Mermaid ER to Oracle

How the Mermaid ER to Oracle conversion works

The Oracle target renders Mermaid diagrams into 19c+ DDL with Oracle's own vocabulary: NUMBER instead of INT, VARCHAR2 instead of VARCHAR, identity clauses instead of SERIAL, and CHECK constraints standing in for enums. Double-quoted identifiers keep entity names exactly as the diagram wrote them.

Mermaid erDiagram is a Markdown-friendly notation rendered natively by GitHub, GitLab, Notion, and Obsidian — which makes it the most common way to keep an ER diagram next to the code it describes.

Entities are blocks of `type name marker` lines (markers: PK, FK, UK, INDEX), and relationships use cardinality arrows like `USER ||--o{ POST : writes` — one-to-many from USER to POST. The converter reads both, plus comment directives for everything the base syntax can't express.

Paste your Mermaid ER source into the editor above and the engine parses it into an intermediate schema — tables, columns, relationships, indexes — then generates Oracle DDL from that model. Anything the diagram syntax can't express (NOT NULL, defaults, enums, CHECK constraints, composite indexes) you add with comment directives like %% ::NN, ::DEFAULT(now()) on the line above a column. The Mermaid ER reference documents the full vocabulary.

Worked example: an e-commerce schema

A complete Mermaid ER diagram and the exact Oracle output the generator produces for it — different from the starter diagram in the editor above, so you can see a second real conversion end to end:

erDiagram
    CUSTOMER ||--o{ ORDER : places
    PRODUCT ||--o{ ORDER : "ordered in"
    CUSTOMER {
        int id PK
        string email UK
        %% ::NN
        string name
        %% ::DEFAULT(now())
        timestamp created_at
    }
    PRODUCT {
        int id PK
        string sku UK
        %% ::CHECK(price > 0)
        decimal price
        %% ::ENUM(draft, active, retired)
        string status
    }
    ORDER {
        int id PK
        int customer_id FK
        int product_id FK
        %% ::DEFAULT(0)
        decimal total
    }

↓ generates

CREATE TABLE "customer" (
    "id" NUMBER GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
    "email" VARCHAR2(255) UNIQUE,
    "name" VARCHAR2(255) NOT NULL,
    "created_at" TIMESTAMP DEFAULT now()
);

CREATE TABLE "order" (
    "id" NUMBER GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
    "customer_id" NUMBER,
    "product_id" NUMBER,
    "total" NUMBER DEFAULT 0
);

CREATE TABLE "product" (
    "id" NUMBER GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
    "sku" VARCHAR2(255) UNIQUE,
    "price" NUMBER CHECK (price > 0),
    "status" VARCHAR2(255) CHECK ("status" IN ('draft', 'active', 'retired'))
);

ALTER TABLE "order"
    ADD CONSTRAINT "fk_order_customer_id"
    FOREIGN KEY ("customer_id") REFERENCES "customer"("id");

ALTER TABLE "order"
    ADD CONSTRAINT "fk_order_product_id"
    FOREIGN KEY ("product_id") REFERENCES "product"("id");

Primary keys came out as NUMBER GENERATED ALWAYS AS IDENTITY — the modern Oracle auto-increment, no sequence-plus-trigger boilerplate. The ::ENUM directive became a CHECK ("status" IN (…)) on a VARCHAR2 column since Oracle has no enum type, and ::CHECK(price > 0) passed straight through. Foreign keys use the same named ALTER TABLE form as PostgreSQL and MySQL.

What the generator emits for Oracle

• CREATE TABLE statements with double-quoted identifiers for Oracle 19c+
• VARCHAR2, NUMBER, and CLOB types sized for Oracle conventions
• CHECK ("col" IN (…)) constraints for ::ENUM(...) columns
• ALTER TABLE … ADD CONSTRAINT foreign keys with deterministic names
• COMMENT ON TABLE / COMMENT ON COLUMN statements

Type mapping: Mermaid ER to Oracle

Diagram column types are logical — the generator maps each one to the idiomatic Oracle type. These are the exact mappings the engine uses:

Keys, relationships, and enums

Primary keys emit as NUMBER columns with PRIMARY KEY constraints. Auto-increment uses Oracle conventions rather than a SERIAL keyword, and composite keys emit a table-level constraint.

Relationships emit `ALTER TABLE … ADD CONSTRAINT fk_<table>_<column>` statements after all tables exist, with ON DELETE actions from the ::RELATIONSHIP directive.

Oracle has no enum type, so `::ENUM(active, inactive)` emits `VARCHAR2(255) CHECK ("status" IN ('active', 'inactive'))` — enforced at the database level via the CHECK constraint.

Frequently asked questions

Related conversions

• Mermaid ER to PostgreSQL
• Mermaid ER to MySQL
• Mermaid ER to SQLite
• Mermaid ER to GORM (Go)
• PlantUML to PostgreSQL
• PlantUML to MySQL
• PlantUML to GORM (Go)
• Diagram diff — compare two schema versions