# Database Schema

## Entity: User

### Table Definition

```sql
CREATE TABLE users (
    id BIGINT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100) NOT NULL UNIQUE,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);
```

### Column Details

| Column | Type | Constraints | Description |
|--------|------|-------------|-------------|
| id | BIGINT | PRIMARY KEY, AUTO_INCREMENT | Unique identifier |
| username | VARCHAR(50) | NOT NULL, UNIQUE | User's unique username |
| email | VARCHAR(100) | NOT NULL, UNIQUE | User's email address |
| created_at | TIMESTAMP | DEFAULT CURRENT_TIMESTAMP | Record creation time |
| updated_at | TIMESTAMP | DEFAULT CURRENT_TIMESTAMP ON UPDATE | Last update time |

### Indexes

| Index Name | Column | Type | Description |
|------------|--------|------|-------------|
| idx_username | username | UNIQUE | Fast username lookup |
| idx_email | email | UNIQUE | Fast email lookup |

### Entity Relationships

```
User (standalone entity, no foreign keys)
```

### JPA Entity Mapping

```java
@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false, unique = true, length = 50)
    private String username;

    @Column(nullable = false, unique = true, length = 100)
    private String email;

    @Column(name = "created_at", updatable = false)
    private LocalDateTime createdAt;

    @Column(name = "updated_at")
    private LocalDateTime updatedAt;
}
```

### Database Support

| Environment | Database | Driver |
|-------------|----------|--------|
| Development | H2 (In-Memory) | org.h2.Driver |
| Test | H2 (In-Memory) | org.h2.Driver |
| Production | MySQL 8.x | com.mysql.cj.jdbc.Driver |
| Production | PostgreSQL 15+ | org.postgresql.Driver |

### Migration Strategy

- **Development**: Auto DDL (Hibernate)
- **Production**: Flyway Migration Scripts
  - Location: `src/main/resources/db/migration/`
  - Naming: `V{version}__{description}.sql`