To create a simple RESTful API using Spring Boot, follow these steps:

1. Install the required tools: Java Development Kit (JDK), Integrated Development Environment (IDE) like IntelliJ IDEA or Eclipse, and Apache Maven.

2. Create a new Spring Boot project using Spring Initializr (https://start.spring.io/). Select "Web", "H2 Database", "JPA" as dependencies, and generate the project.

3. Extract the generated project and open it in your IDE.

4. Create an Employee entity class with fields id, name, email, and department. Add appropriate annotations for JPA to map this class to a database table.

```java
// src/main/java/com/example/demo/entity/Employee.java

package com.example.demo.entity;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String name;
    private String email;
    private String department;

    // Getters and setters
}
```

5. Create a repository interface that extends JpaRepository for performing CRUD operations on the Employee entity.

```java
// src/main/java/com/example/demo/repository/EmployeeRepository.java

package com.example.demo.repository;

import com.example.demo.entity.Employee;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface EmployeeRepository extends JpaRepository<Employee, Long> {
}
```

6. Create a service class for handling business logic related to Employee operations.

```java
// src/main/java/com/example/demo/service/EmployeeService.java

package com.example.demo.service;

import com.example.demo.entity.Employee;
import com.example.demo.repository.EmployeeRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class EmployeeService {

    @Autowired
    private EmployeeRepository employeeRepository;

    public Employee createEmployee(Employee employee) {
        return employeeRepository.save(employee);
    }

    public List<Employee> getAllEmployees() {
        return employeeRepository.findAll();
    }

    public Employee getEmployeeById(Long id) {
        return employeeRepository.findById(id).orElse(null);
    }

    public Employee updateEmployee(Long id, Employee updatedEmployee) {
        Employee existingEmployee = employeeRepository.findById(id).orElse(null);
        if (existingEmployee != null) {
            existingEmployee.setName(updatedEmployee.getName());
            existingEmployee.setEmail(updatedEmployee.getEmail());
            existingEmployee.setDepartment(updatedEmployee.getDepartment());
            return employeeRepository.save(existingEmployee);
        }
        return null;
    }

    public void deleteEmployee(Long id) {
        employeeRepository.deleteById(id);
    }
}
```

7. Create a controller class for handling HTTP requests related to Employee operations.

```java
// src/main/java/com/example/demo/controller/EmployeeController.java

package com.example.demo.controller;

import com.example.demo.entity.Employee;
import com.example.demo.service.EmployeeService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/api/employees")
public class EmployeeController {

    @Autowired
    private EmployeeService employeeService;

    @PostMapping
    public Employee createEmployee(@RequestBody Employee employee) {
        return employeeService.createEmployee(employee);
    }

    @GetMapping
    public List<Employee> getAllEmployees() {
        return employeeService.getAllEmployees();
    }

    @GetMapping("/{id}")
    public Employee getEmployeeById(@PathVariable Long id) {
        return employeeService.getEmployeeById(id);
    }

    @PutMapping("/{id}")
    public Employee updateEmployee(@PathVariable Long id, @RequestBody Employee updatedEmployee) {
        return employeeService.updateEmployee(id, updatedEmployee);
    }

    @DeleteMapping("/{id}")
    public void deleteEmployee(@PathVariable Long id) {
        employeeService.deleteEmployee(id);
    }
}
```

8. Configure the H2 database in `application.properties` file.

```properties
# src/main/resources/application.properties

spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=

spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
spring.jpa.hibernate.ddl-auto=update

spring.h2.console.enabled=true
spring.h2.console.path=/h2-console
```

9. Run the application by executing the main class (DemoApplication.java) or using the command `mvn spring-boot:run`.

10. Test the API endpoints using a tool like Postman or curl.

You have now created a simple RESTful API with CRUD operations for an Employee entity using Spring Boot and an in-memory H2 database.
Spring Boot를 사용하여 간단한 RESTful API를 만들려면 다음 단계를 따르세요.

1. 필요한 도구를 설치합니다: Java 개발 키트(JDK), 통합 개발 환경(IDE)(예: IntelliJ IDEA 또는 Eclipse), Apache Maven.

2. Spring Initializr(https://start.spring.io/)를 사용하여 새 Spring Boot 프로젝트를 생성합니다. 종속성으로 "Web", "H2 Database", "JPA"를 선택하고 프로젝트를 생성합니다.

3. 생성된 프로젝트를 압축을 풀고 IDE에서 엽니다.

4. 아이디, 이름, 이메일, 부서 필드를 사용하여 Employee 엔티티 클래스를 생성합니다. 이 클래스를 데이터베이스 테이블에 매핑할 수 있도록 적절한 주석을 추가합니다.

```java
// src/main/java/com/example/demo/entity/Employee.java

package com.example.demo.entity;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String name;
    private String email;
    private String department;

    // Getters and setters
}
```

5. Employee 엔터티에서 CRUD 작업을 수행하기 위해 JpaRepository를 확장하는 리포지토리 인터페이스를 생성합니다.

```java
// src/main/java/com/example/demo/repository/EmployeeRepository.java

package com.example.demo.repository;

import com.example.demo.entity.Employee;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface EmployeeRepository extends JpaRepository<Employee, Long> {
}
```

6. 직원 운영과 관련된 비즈니스 로직을 처리하는 서비스 클래스를 생성합니다.

```java
// src/main/java/com/example/demo/service/EmployeeService.java

package com.example.demo.service;

import com.example.demo.entity.Employee;
import com.example.demo.repository.EmployeeRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class EmployeeService {

    @Autowired
    private EmployeeRepository employeeRepository;

    public Employee createEmployee(Employee employee) {
        return employeeRepository.save(employee);
    }

    public List<Employee> getAllEmployees() {
        return employeeRepository.findAll();
    }

    public Employee getEmployeeById(Long id) {
        return employeeRepository.findById(id).orElse(null);
    }

    public Employee updateEmployee(Long id, Employee updatedEmployee) {
        Employee existingEmployee = employeeRepository.findById(id).orElse(null);
        if (existingEmployee != null) {
            existingEmployee.setName(updatedEmployee.getName());
            existingEmployee.setEmail(updatedEmployee.getEmail());
            existingEmployee.setDepartment(updatedEmployee.getDepartment());
            return employeeRepository.save(existingEmployee);
        }
        return null;
    }

    public void deleteEmployee(Long id) {
        employeeRepository.deleteById(id);
    }
}
```

7. Employee 작업과 관련된 HTTP 요청을 처리하기 위한 컨트롤러 클래스를 생성합니다.

```java
// src/main/java/com/example/demo/controller/EmployeeController.java

package com.example.demo.controller;

import com.example.demo.entity.Employee;
import com.example.demo.service.EmployeeService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/api/employees")
public class EmployeeController {

    @Autowired
    private EmployeeService employeeService;

    @PostMapping
    public Employee createEmployee(@RequestBody Employee employee) {
        return employeeService.createEmployee(employee);
    }

    @GetMapping
    public List<Employee> getAllEmployees() {
        return employeeService.getAllEmployees();
    }

    @GetMapping("/{id}")
    public Employee getEmployeeById(@PathVariable Long id) {
        return employeeService.getEmployeeById(id);
    }

    @PutMapping("/{id}")
    public Employee updateEmployee(@PathVariable Long id, @RequestBody Employee updatedEmployee) {
        return employeeService.updateEmployee(id, updatedEmployee);
    }

    @DeleteMapping("/{id}")
    public void deleteEmployee(@PathVariable Long id) {
        employeeService.deleteEmployee(id);
    }
}
```

8. `application.properties` 파일에서 H2 데이터베이스를 구성합니다.

```properties
# src/main/resources/application.properties

spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=

spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
spring.jpa.hibernate.ddl-auto=update

spring.h2.console.enabled=true
spring.h2.console.path=/h2-console
```

9. 메인 클래스(DemoApplication.java)를 실행하거나 `mvn spring-boot:run` 명령을 사용하여 애플리케이션을 실행합니다.

10. Postman 또는 curl과 같은 도구를 사용하여 API 엔드포인트를 테스트합니다.

이제 Spring Boot와 인메모리 H2 데이터베이스를 사용하여 Employee 엔티티에 대한 CRUD 작업이 포함된 간단한 RESTful API를 만들었습니다.