schema-designer

Schema Designer

Help users design database schemas, create tables, and model data relationships.

When to Use

Activate when user asks:

• "Create a table for storing orders"
• "Design a schema for a blog"
• "Add a column to track user preferences"
• "How should I model this relationship?"

Workflow

1. Understand Requirements

Ask clarifying questions:

• What data needs to be stored?
• What are the relationships between entities?
• What queries will be common?
• What's the expected data volume?

2. Check Existing Schema

whodb_tables() → See what already exists
whodb_columns(table="related_table") → Understand existing structure

3. Design the Schema

Follow database design principles:

• Normalize to reduce redundancy
• Use appropriate data types
• Define primary keys
• Establish foreign key relationships
• Add indexes for common queries

4. Generate DDL

Provide CREATE TABLE statements with explanations.

Data Type Guidelines

Identifiers

Use Case	PostgreSQL	MySQL	SQLite
Auto-increment ID	SERIAL / BIGSERIAL	INT AUTO_INCREMENT	INTEGER PRIMARY KEY
UUID	UUID	CHAR(36)	TEXT

Text

Use Case	PostgreSQL	MySQL	SQLite
Short text (<255)	VARCHAR(n)	VARCHAR(n)	TEXT
Long text	TEXT	TEXT	TEXT
Fixed length	CHAR(n)	CHAR(n)	TEXT

Numbers

Use Case	PostgreSQL	MySQL	SQLite
Integer	INTEGER	INT	INTEGER
Big integer	BIGINT	BIGINT	INTEGER
Decimal (money)	NUMERIC(10,2)	DECIMAL(10,2)	REAL
Float	REAL	FLOAT	REAL

Dates

Use Case	PostgreSQL	MySQL	SQLite
Date only	DATE	DATE	TEXT
Timestamp	TIMESTAMP	DATETIME	TEXT
With timezone	TIMESTAMPTZ	TIMESTAMP	TEXT

Boolean

PostgreSQL	MySQL	SQLite
BOOLEAN	TINYINT(1)	INTEGER

Common Patterns

Users Table

CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    email VARCHAR(255) NOT NULL UNIQUE,
    password_hash VARCHAR(255) NOT NULL,
    name VARCHAR(100),
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

CREATE INDEX idx_users_email ON users(email);

One-to-Many (Orders → Order Items)

CREATE TABLE orders (
    id SERIAL PRIMARY KEY,
    user_id INTEGER NOT NULL REFERENCES users(id),
    status VARCHAR(20) DEFAULT 'pending',
    total NUMERIC(10,2),
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

CREATE TABLE order_items (
    id SERIAL PRIMARY KEY,
    order_id INTEGER NOT NULL REFERENCES orders(id) ON DELETE CASCADE,
    product_id INTEGER NOT NULL REFERENCES products(id),
    quantity INTEGER NOT NULL DEFAULT 1,
    unit_price NUMERIC(10,2) NOT NULL
);

CREATE INDEX idx_order_items_order ON order_items(order_id);

Many-to-Many (Users ↔ Roles)

CREATE TABLE roles (
    id SERIAL PRIMARY KEY,
    name VARCHAR(50) NOT NULL UNIQUE
);

CREATE TABLE user_roles (
    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    role_id INTEGER NOT NULL REFERENCES roles(id) ON DELETE CASCADE,
    PRIMARY KEY (user_id, role_id)
);

Soft Delete Pattern

CREATE TABLE posts (
    id SERIAL PRIMARY KEY,
    title VARCHAR(255) NOT NULL,
    content TEXT,
    deleted_at TIMESTAMP NULL,  -- NULL = not deleted
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

-- Query active posts
SELECT * FROM posts WHERE deleted_at IS NULL;

Audit Trail Pattern

CREATE TABLE audit_log (
    id SERIAL PRIMARY KEY,
    table_name VARCHAR(50) NOT NULL,
    record_id INTEGER NOT NULL,
    action VARCHAR(10) NOT NULL,  -- INSERT, UPDATE, DELETE
    old_values JSONB,
    new_values JSONB,
    user_id INTEGER REFERENCES users(id),
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

CREATE INDEX idx_audit_table_record ON audit_log(table_name, record_id);

Best Practices

1. Always define PRIMARY KEY - Every table needs one
2. Use foreign keys - Enforce referential integrity
3. Add NOT NULL - Unless the column is truly optional
4. Create indexes - On foreign keys and frequently queried columns
5. Use appropriate types - Don't store numbers as strings
6. Add timestamps - created_at and updated_at are almost always useful
7. Name consistently - user_id not userId or UserID
8. Avoid reserved words - Don't name columns order, user, group

Migration Safety

When modifying existing tables:

-- Safe: Adding nullable column
ALTER TABLE users ADD COLUMN phone VARCHAR(20);

-- Safe: Adding column with default
ALTER TABLE users ADD COLUMN active BOOLEAN DEFAULT true;

-- Caution: Adding NOT NULL (requires default or backfill)
ALTER TABLE users ADD COLUMN status VARCHAR(20) NOT NULL DEFAULT 'active';

-- Caution: Dropping column (data loss)
ALTER TABLE users DROP COLUMN old_column;

-- Caution: Changing type (may fail on existing data)
ALTER TABLE users ALTER COLUMN age TYPE INTEGER;