spring boot 连表查询返回树状结构的数据

对于返回树状结构的数据，可以使用递归查询来实现。在Spring Boot中，可以使用JPA或MyBatis等框架完成数据库操作。以下是一个示例，假设我们有一个Department和Employee两个实体类，它们之间是一对多的关系，一个部门可以有多个员工。

首先，定义一个DTO类，用于存储树状结构数据：

public class DepartmentDTO {
    private Long id;
    private String name;
    private List<DepartmentDTO> children;
    private List<Employee> employees;
    // getter and setter
}

然后，定义一个DepartmentRepository接口：

public interface DepartmentRepository extends JpaRepository<Department, Long> {
    @Query("select d from Department d where d.parent is null")
    List<Department> findRootDepartments();

    @Query("select d from Department d where d.parent.id = :parentId")
    List<Department> findChildrenByParentId(@Param("parentId") Long parentId);
}

接下来，定义一个DepartmentService类，用于实现递归查询：

@Service
public class DepartmentService {
    @Autowired
    private DepartmentRepository departmentRepository;

    public List<DepartmentDTO> getDepartmentTree() {
        List<Department> rootDepartments = departmentRepository.findRootDepartments();
        List<DepartmentDTO> departmentDTOS = new ArrayList<>();
        for (Department rootDepartment : rootDepartments) {
            DepartmentDTO departmentDTO = new DepartmentDTO();
            departmentDTO.setId(rootDepartment.getId());
            departmentDTO.setName(rootDepartment.getName());
            departmentDTO.setEmployees(rootDepartment.getEmployees());
            departmentDTO.setChildren(getChildren(rootDepartment.getId()));
            departmentDTOS.add(departmentDTO);
        }
        return departmentDTOS;
    }

    private List<DepartmentDTO> getChildren(Long parentId) {
        List<Department> children = departmentRepository.findChildrenByParentId(parentId);
        List<DepartmentDTO> departmentDTOS = new ArrayList<>();
        for (Department child : children) {
            DepartmentDTO departmentDTO = new DepartmentDTO();
            departmentDTO.setId(child.getId());
            departmentDTO.setName(child.getName());
            departmentDTO.setEmployees(child.getEmployees());
            departmentDTO.setChildren(getChildren(child.getId()));
            departmentDTOS.add(departmentDTO);
        }
        return departmentDTOS;
    }
}

最后，在Controller中调用Service方法并返回数据即可：

@RestController
@RequestMapping("/departments")
public class DepartmentController {
    @Autowired
    private DepartmentService departmentService;

    @GetMapping("/tree")
    public List<DepartmentDTO> getDepartmentTree() {
        return departmentService.getDepartmentTree();
    }
}

这样就可以返回树状结构的数据了。需要注意的是，如果数据量比较大，递归查询可能会导致性能问题，可以考虑使用其他方式实现。