database-schema-design

Database schema design for PostgreSQL/MySQL with normalization, relationships, constraints. Use for new databases, schema reviews, migrations, or encountering missing PKs/FKs, wrong data types, premature denormalization, EAV anti-pattern.

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name	database-schema-design
description	Database schema design for PostgreSQL/MySQL with normalization, relationships, constraints. Use for new databases, schema reviews, migrations, or encountering missing PKs/FKs, wrong data types, premature denormalization, EAV anti-pattern.
license	MIT
metadata	{"keywords":"database schema, schema design, database normalization, 1nf 2nf 3nf, primary key, foreign key, database relationships, one to many, many to many, data types postgresql, constraints check, audit columns, soft delete, database best practices, schema patterns, database anti-patterns, missing primary key, no foreign key, varchar max, denormalization, entity relationship, composite key, uuid vs bigserial, timestamptz"}

database-schema-design

Comprehensive database schema design patterns for PostgreSQL and MySQL with normalization, relationships, constraints, and error prevention.

Quick Start (10 Minutes)

Step 1: Choose your schema pattern from templates:

# Basic schema with users, products, orders
cat templates/basic-schema.sql

# Relationship patterns (1:1, 1:M, M:M)
cat templates/relationships.sql

# Constraint examples
cat templates/constraints.sql

# Audit patterns
cat templates/audit-columns.sql

CREATE TABLE your_table (
  -- Primary key (required)
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),

  -- Business columns with proper types
  name VARCHAR(200) NOT NULL,  -- Use appropriate lengths

  -- Audit columns (always include)
  created_at TIMESTAMPTZ DEFAULT NOW() NOT NULL,
  updated_at TIMESTAMPTZ DEFAULT NOW() NOT NULL
);

Rule	Reason
Every table has PRIMARY KEY	Ensures row uniqueness, enables relationships
Foreign keys defined explicitly	Enforces referential integrity, prevents orphans
Index all foreign keys	Prevents slow JOINs, critical for performance
NOT NULL on required fields	Data integrity, prevents NULL pollution
Audit columns (created_at, updated_at)	Track changes, debugging, compliance
Appropriate data types	Storage efficiency, validation, indexing
Check constraints for enums	Enforces valid values at database level
ON DELETE/UPDATE rules specified	Prevents accidental data loss or orphans

Anti-Pattern	Why It's Bad
VARCHAR(MAX) everywhere	Wastes space, slows indexes, no validation
Dates as VARCHAR	No date math, no validation, sorting broken
Missing foreign keys	No referential integrity, orphaned records
Premature denormalization	Hard to maintain, data anomalies
EAV (Entity-Attribute-Value)	Query complexity, no type safety, slow
Polymorphic associations	No foreign key integrity, complex queries
Circular dependencies	Impossible to populate, breaks CASCADE
No indexes on foreign keys	Extremely slow JOINs, performance killer

-- ❌ Bad
CREATE TABLE users (
  email VARCHAR(255),
  name VARCHAR(100)
);

-- ✅ Good
CREATE TABLE users (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  email VARCHAR(255) UNIQUE NOT NULL,
  name VARCHAR(100) NOT NULL
);

-- ❌ Bad
CREATE TABLE orders (
  id UUID PRIMARY KEY,
  user_id UUID  -- No constraint!
);

-- ✅ Good
CREATE TABLE orders (
  id UUID PRIMARY KEY,
  user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE
);

-- Index the foreign key
CREATE INDEX idx_orders_user_id ON orders(user_id);

-- ❌ Bad
CREATE TABLE products (
  name VARCHAR(MAX),
  sku VARCHAR(MAX),
  status VARCHAR(MAX)
);

-- ✅ Good
CREATE TABLE products (
  name VARCHAR(200) NOT NULL,
  sku VARCHAR(50) UNIQUE NOT NULL,
  status VARCHAR(20) NOT NULL
    CHECK (status IN ('draft', 'active', 'archived'))
);

-- ❌ Bad
CREATE TABLE events (
  event_date VARCHAR(50)  -- '2025-12-15' or 'Dec 15, 2025'?
);

-- ✅ Good
CREATE TABLE events (
  event_date DATE NOT NULL,  -- Validated, sortable
  event_time TIMESTAMPTZ     -- With timezone
);

-- Always index foreign keys
CREATE TABLE order_items (
  order_id UUID NOT NULL REFERENCES orders(id),
  product_id UUID NOT NULL REFERENCES products(id)
);

-- ✅ Required indexes
CREATE INDEX idx_order_items_order_id ON order_items(order_id);
CREATE INDEX idx_order_items_product_id ON order_items(product_id);

-- ❌ Bad
CREATE TABLE products (
  id UUID PRIMARY KEY,
  name VARCHAR(200)
);

-- ✅ Good
CREATE TABLE products (
  id UUID PRIMARY KEY,
  name VARCHAR(200) NOT NULL,
  created_at TIMESTAMPTZ DEFAULT NOW() NOT NULL,
  updated_at TIMESTAMPTZ DEFAULT NOW() NOT NULL
);

-- Auto-update trigger (PostgreSQL)
CREATE TRIGGER products_updated_at
BEFORE UPDATE ON products
FOR EACH ROW
EXECUTE FUNCTION update_updated_at_column();

-- ❌ Bad (EAV)
CREATE TABLE product_attributes (
  product_id UUID,
  attribute_name VARCHAR(100),  -- 'color', 'size', 'price'
  attribute_value TEXT           -- Everything as text!
);

-- ✅ Good (Structured + JSONB)
CREATE TABLE products (
  id UUID PRIMARY KEY,
  name VARCHAR(200) NOT NULL,
  price DECIMAL(10,2) NOT NULL,  -- Required fields as columns
  color VARCHAR(50),              -- Common attributes as columns
  size VARCHAR(20),
  attributes JSONB                -- Optional/dynamic attributes
);

-- Index JSONB
CREATE INDEX idx_products_attributes ON products USING GIN(attributes);

Pattern	Use Case	Template
Basic CRUD	Standard users/products/orders	`templates/basic-schema.sql`
One-to-One	User → Profile	`templates/relationships.sql` (lines 7-17)
One-to-Many	User → Orders	`templates/relationships.sql` (lines 23-34)
Many-to-Many	Students ↔ Courses	`templates/relationships.sql` (lines 40-60)
Hierarchical	Categories tree, org chart	`templates/relationships.sql` (lines 66-83)
Soft Delete	Mark deleted, keep history	`templates/audit-columns.sql` (lines 55-80)
Versioning	Track changes over time	`templates/audit-columns.sql` (lines 86-108)
Multi-Tenant	Isolated data per organization	`references/schema-design-patterns.md` (lines 228-258)

Form	Rule	Example
1NF	Atomic values, no repeating groups	`phone1, phone2` → `phones` table
2NF	1NF + no partial dependencies	Composite key dependency → separate table
3NF	2NF + no transitive dependencies	`user.city` → `city.id` reference
BCNF	3NF + every determinant is candidate key	Rare edge cases
4NF	BCNF + no multi-valued dependencies	Complex many-to-many
5NF	4NF + no join dependencies	Very rare, academic

-- Primary Keys
id UUID PRIMARY KEY DEFAULT gen_random_uuid()
-- OR for performance-critical:
id BIGSERIAL PRIMARY KEY

-- Text
name VARCHAR(200) NOT NULL
description TEXT
code CHAR(10)  -- Fixed-length codes only

-- Numbers
price DECIMAL(10,2) NOT NULL  -- Money: NEVER use FLOAT
quantity INT NOT NULL
rating DECIMAL(3,2)  -- 0.00 to 9.99

-- Dates/Times
created_at TIMESTAMPTZ DEFAULT NOW() NOT NULL  -- With timezone
event_date DATE
duration INTERVAL

-- Boolean
is_active BOOLEAN DEFAULT true NOT NULL

-- JSON
attributes JSONB  -- Binary, faster, indexable

-- Enum Alternative (preferred over ENUM type)
status VARCHAR(20) NOT NULL
  CHECK (status IN ('draft', 'active', 'archived'))

-- MySQL doesn't have:
TIMESTAMPTZ  -- Use TIMESTAMP (stored as UTC)
gen_random_uuid()  -- Use UUID() function
JSONB  -- Use JSON (same performance in 8.0+)

-- MySQL equivalent:
id CHAR(36) PRIMARY KEY DEFAULT (UUID())
-- OR:
id BIGINT AUTO_INCREMENT PRIMARY KEY

created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL
attributes JSON

CREATE TABLE users (
  email VARCHAR(MAX),           -- Issue: No primary key, VARCHAR(MAX)
  password VARCHAR(MAX),
  created VARCHAR(50)           -- Issue: Date as string
);

CREATE TABLE orders (
  id UUID PRIMARY KEY,
  user_email VARCHAR(MAX),      -- Issue: No foreign key
  total VARCHAR(20),            -- Issue: Money as string
  status VARCHAR(MAX)           -- Issue: No validation
);

CREATE TABLE users (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  email VARCHAR(255) UNIQUE NOT NULL,
  password_hash VARCHAR(255) NOT NULL,
  created_at TIMESTAMPTZ DEFAULT NOW() NOT NULL,
  updated_at TIMESTAMPTZ DEFAULT NOW() NOT NULL
);

CREATE TABLE orders (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
  total DECIMAL(10,2) NOT NULL CHECK (total >= 0),
  status VARCHAR(20) NOT NULL DEFAULT 'pending'
    CHECK (status IN ('pending', 'processing', 'shipped', 'delivered', 'canceled')),
  created_at TIMESTAMPTZ DEFAULT NOW() NOT NULL,
  updated_at TIMESTAMPTZ DEFAULT NOW() NOT NULL
);

CREATE INDEX idx_orders_user_id ON orders(user_id);
CREATE INDEX idx_orders_status ON orders(status);