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设计图如图：

直接上代码

数据库中的格式：

JAVA实体类：

@Data
public class SysAreaZoningDO {private Long districtId;private Long parentId;private String districtName;private List<SysAreaZoningDO> children;
}

MapperSQL语句：

<select id="getAllArea" resultType="cn.iocoder.saas.module.bank.dal.dataobject.syszoning.SysAreaZoningDO">WITH Provinces AS (SELECT *FROM sys_zoningWHERE parent_id = 0),Cities AS (SELECT *FROM sys_zoningWHERE parent_id IN (SELECT id FROM Provinces)),Districts AS (SELECT *FROM sys_zoningWHERE parent_id IN (SELECT id FROM Cities))SELECTp.id AS districtId,p.parent_id AS parentId,p.name AS districtName,NULL AS order_city,NULL AS order_area,'province' AS levelFROM Provinces pUNION ALLSELECTc.id AS districtId,c.parent_id AS parentId,c.name AS districtName,c.order_city AS order_city,NULL AS order_area,'city' AS levelFROM Cities cUNION ALLSELECTd.id AS districtId,d.parent_id AS parentId,d.name AS districtName,c.order_city AS order_city,d.order_area AS order_area,'district' AS levelFROM Districts dJOIN Cities c ON d.parent_id = c.idORDER BY order_city DESC,order_area ASC;</select>

Service层实现方法：

@Overridepublic List<SysAreaZoningDO> getAllArea() {// 获取扁平化的所有区域数据List<SysAreaZoningDO> allAreas = userAddressMapper.getAllArea();Map<Long, SysAreaZoningDO> areaMap = new HashMap<>();List<SysAreaZoningDO> provinces = new ArrayList<>();for (SysAreaZoningDO area : allAreas) {areaMap.put(area.getDistrictId(), area);if (area.getParentId() == 0) {provinces.add(area);}}for (SysAreaZoningDO area : allAreas) {if (area.getParentId() != 0) {SysAreaZoningDO parent = areaMap.get(area.getParentId());if (parent != null) {if (parent.getChildren() == null) {parent.setChildren(new ArrayList<>());}parent.getChildren().add(area);}}}return provinces;}

返回格式：