SQL Schema Design Best Practices

Design database schema for [APPLICATION]:

**Naming Conventions**:
- Tables: plural, snake_case (users, order_items)
- Columns: singular, snake_case (user_id, created_at)
- Primary keys: id (integer, auto-increment)
- Foreign keys: [table_singular]_id

**Standard Columns** (all tables):
- id: Primary key
- created_at: Timestamp
- updated_at: Timestamp
- deleted_at: Soft deletes (optional)

**Relationships**:
- One-to-Many: Foreign key in child table
- Many-to-Many: Junction table
- One-to-One: Foreign key with UNIQUE constraint

**Indexes**:
- Primary key (automatic)
- Foreign keys
- Frequently queried columns
- Composite indexes for multi-column queries

**Data Types**:
- Use appropriate sizes (VARCHAR vs TEXT)
- ENUM for fixed options
- JSON for flexible data (use sparingly)
- UUID for distributed systems

**Constraints**:
- NOT NULL where appropriate
- UNIQUE for natural keys
- CHECK for validation
- DEFAULT values

**Normalization**:
- 3NF for OLTP
- Denormalize for read-heavy (with care)
- Avoid EAV anti-pattern

**Example Schema**:
```sql
CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  email VARCHAR(255) UNIQUE NOT NULL,
  password_hash VARCHAR(255) NOT NULL,
  created_at TIMESTAMP DEFAULT NOW(),
  updated_at TIMESTAMP DEFAULT NOW()
);

CREATE INDEX idx_users_email ON users(email);
```