设备，楼，层三者依次为父子节点，请设计出MySQL数据库表，后端java语言如何去查询并且返回给前端，前端用<el-tree>渲染出来，写出全部代码

好的，我会根据你的要求回答你的问题。首先，我会设计MySQL数据库的表： CREATE TABLE `device` ( `id` int(10) UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '设备ID', `name` varchar(50) COLLATE utf8mb4_unicode_ci NOT NULL COMMENT '设备名称', `parent_id` int(10) UNSIGNED NOT NULL DEFAULT '0' COMMENT '父设备ID', PRIMARY KEY (`id`), KEY `parent_id` (`parent_id`), CONSTRAINT `device_ibfk_1` FOREIGN KEY (`parent_id`) REFERENCES `device` (`id`) ON DELETE CASCADE ON UPDATE CASCADE ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='设备表'; CREATE TABLE `building` ( `id` int(10) UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '楼ID', `name` varchar(50) COLLATE utf8mb4_unicode_ci NOT NULL COMMENT '楼名称', PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='楼表'; CREATE TABLE `floor` ( `id` int(10) UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '层ID', `name` varchar(50) COLLATE utf8mb4_unicode_ci NOT NULL COMMENT '层名称', `building_id` int(10) UNSIGNED NOT NULL COMMENT '所属楼ID', PRIMARY KEY (`id`), KEY `building_id` (`building_id`), CONSTRAINT `floor_ibfk_1` FOREIGN KEY (`building_id`) REFERENCES `building` (`id`) ON DELETE CASCADE ON UPDATE CASCADE ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='层表'; 这样我们就有了设备表，楼表和层表，其中设备表中的parent_id是指父设备的ID，如果这个设备是顶级设备，则parent_id设置为0。另外，我们通过外键来建立设备、楼和层的关系，方便后面的查询。 接下来，我会给出后端Java代码： /** * 获取所有设备、楼、层节点 * * @return 节点列表 */ @GetMapping("/nodes") public List<TreeNode> getNodes() { List<Device> devices = deviceService.getAllDevices(); List<Building> buildings = buildingService.getAllBuildings(); List<Floor> floors = floorService.getAllFloors(); // 构造设备节点 List<TreeNode> deviceNodes = new ArrayList<>(); for (Device device : devices) { TreeNode node = new TreeNode(); node.setId(device.getId()); node.setLabel(device.getName()); node.setParentId(device.getParentId()); node.setType(TreeNode.TYPE_DEVICE); deviceNodes.add(node); } // 构造楼节点 List<TreeNode> buildingNodes = new ArrayList<>(); for (Building building : buildings) { TreeNode node = new TreeNode(); node.setId(building.getId()); node.setLabel(building.getName()); node.setType(TreeNode.TYPE_BUILDING); buildingNodes.add(node); } // 构造层节点 List<TreeNode> floorNodes = new ArrayList<>(); for (Floor floor : floors) { TreeNode node = new TreeNode(); node.setId(floor.getId()); node.setLabel(floor.getName()); node.setParentId(floor.getBuildingId()); node.setType(TreeNode.TYPE_FLOOR); floorNodes.add(node); } // 构造树结构 List<TreeNode> treeNodes = new ArrayList<>(); treeNodes.addAll(deviceNodes); treeNodes.addAll(buildingNodes); treeNodes.addAll(floorNodes); for (TreeNode treeNode : treeNodes) { Long parentId = treeNode.getParentId(); if (parentId != null && parentId > 0) { TreeNode parentNode = treeNodes.stream() .filter(node -> node.getId().equals(parentId)) .findFirst().orElse(null); treeNode.setParent(parentNode); if (parentNode != null) { parentNode.addChild(treeNode); } } } // 返回根节点 return treeNodes.stream() .filter(node -> node.getParent() == null) .collect(Collectors.toList()); } 对于前端部分，我们可以使用Vue.js和element-ui框架来实现。我们可以使用<el-tree>组件来展示树形结构，具体代码如下： <template> <el-tree :data="nodes" :expand-on-click-node="false"></el-tree> </template> <script> import axios from 'axios' export default { data() { return { nodes: [] } }, mounted() { axios.get('/api/nodes').then(res => { this.nodes = res.data }) } } </script> 这样就可以将后端返回的树形结构渲染出来了。 完整代码如下： Java： public class Device { private Long id; private String name; private Long parentId; // 省略getter和setter } public class Building { private Long id; private String name; // 省略getter和setter } public class Floor { private Long id; private String name; private Long buildingId; // 省略getter和setter } public class TreeNode { public static final String TYPE_DEVICE = "device"; public static final String TYPE_BUILDING = "building"; public static final String TYPE_FLOOR = "floor"; private Long id; private String label; private Long parentId; private String type; private TreeNode parent; private List<TreeNode> children = new ArrayList<>(); // 省略getter和setter public void addChild(TreeNode child) { children.add(child); } } @RestController @RequestMapping("/api") public class TreeController { @Autowired private DeviceService deviceService; @Autowired private BuildingService buildingService; @Autowired private FloorService floorService; /** * 获取所有设备、楼、层节点 * * @return 节点列表 */ @GetMapping("/nodes") public List<TreeNode> getNodes() { List<Device> devices = deviceService.getAllDevices(); List<Building> buildings = buildingService.getAllBuildings(); List<Floor> floors = floorService.getAllFloors(); // 构造设备节点 List<TreeNode> deviceNodes = new ArrayList<>(); for (Device device : devices) { TreeNode node = new TreeNode(); node.setId(device.getId()); node.setLabel(device.getName()); node.setParentId(device.getParentId()); node.setType(TreeNode.TYPE_DEVICE); deviceNodes.add(node); } // 构造楼节点 List<TreeNode> buildingNodes = new ArrayList<>(); for (Building building : buildings) { TreeNode node = new TreeNode(); node.setId(building.getId()); node.setLabel(building.getName()); node.setType(TreeNode.TYPE_BUILDING); buildingNodes.add(node); } // 构造层节点 List<TreeNode> floorNodes = new ArrayList<>(); for (Floor floor : floors) { TreeNode node = new TreeNode(); node.setId(floor.getId()); node.setLabel(floor.getName()); node.setParentId(floor.getBuildingId()); node.setType(TreeNode.TYPE_FLOOR); floorNodes.add(node); } // 构造树结构 List<TreeNode> treeNodes = new ArrayList<>(); treeNodes.addAll(deviceNodes); treeNodes.addAll(buildingNodes); treeNodes.addAll(floorNodes); for (TreeNode treeNode : treeNodes) { Long parentId = treeNode.getParentId(); if (parentId != null && parentId > 0) { TreeNode parentNode = treeNodes.stream() .filter(node -> node.getId().equals(parentId)) .findFirst().orElse(null); treeNode.setParent(parentNode); if (parentNode != null) { parentNode.addChild(treeNode); } } } // 返回根节点 return treeNodes.stream() .filter(node -> node.getParent() == null) .collect(Collectors.toList()); } } Vue.js： <template> <el-tree :data="nodes" :expand-on-click-node="false"></el-tree> </template> <script> import axios from 'axios' export default { data() { return { nodes: [] } }, mounted() { axios.get('/api/nodes').then(res => { this.nodes = res.data }) } } </script> 注：这是一个示例，实际使用时需要根据具体需求进行适当调整。