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+# 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`