通过mybatis递归获取树结构数据

sql：

/*
 Navicat Premium Data Transfer

 Source Server         : localhost
 Source Server Type    : MySQL
 Source Server Version : 50724
 Source Host           : localhost:3306
 Source Schema         : local_test

 Target Server Type    : MySQL
 Target Server Version : 50724
 File Encoding         : 65001

 Date: 25/03/2020 17:23:37
*/

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for citys
-- ----------------------------
DROP TABLE IF EXISTS `citys`;
CREATE TABLE `citys`  (
  `id` int(5) NOT NULL AUTO_INCREMENT COMMENT 'id',
  `pid` int(5) NULL DEFAULT NULL COMMENT '父id',
  `city` varchar(100) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL COMMENT '城市',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 8 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of citys
-- ----------------------------
INSERT INTO `citys` VALUES (1, 0, '湖南');
INSERT INTO `citys` VALUES (2, 0, '江西');
INSERT INTO `citys` VALUES (3, 0, '广东');
INSERT INTO `citys` VALUES (4, 1, '长沙');
INSERT INTO `citys` VALUES (5, 1, '常德');
INSERT INTO `citys` VALUES (6, 1, '岳阳');
INSERT INTO `citys` VALUES (7, 2, '萍乡');

SET FOREIGN_KEY_CHECKS = 1;

先上代码：

这个是pojo类

import com.baomidou.mybatisplus.annotation.TableField;
import com.baomidou.mybatisplus.annotation.TableId;
import com.baomidou.mybatisplus.annotation.TableName;
import lombok.Data;

import java.util.List;


@TableName("citys")
@Data
public class Citys {
    @TableId("id")
    private Integer id;

    @TableField("pid")
    private Integer pId;


    @TableField("city")
    private String city;


    @TableField(exist = false)
    private List<Citys> list;
}

这个是Mapper

import com.bjsxt.entity.Citys;

import java.util.List;

public interface CitysMapper {

    List<Citys> getAll();

    List<Citys> getCitys();
}

这个是mapper.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.demo.mapper.CitysMapper">

    <resultMap id="BaseResultMap" type="com.bjsxt.entity.Citys">
        <id column="id" jdbcType="BIGINT" property="id"/>
        <result column="pid" jdbcType="BIGINT" property="pId"/>
        <result column="city" jdbcType="VARCHAR" property="city"/>
        <collection property="list" ofType="citys" select="getCitysByPid" column="{pid=id}"/>

    </resultMap>

    <sql id="Base_Column_List">
        id,pid,city
    </sql>


    <select id="getAll" resultType="citys">
        select * from citys

    </select>

    <select id="getCitys" resultMap="BaseResultMap">
        select
        <include refid="Base_Column_List"/>
        from citys
        where pid=0

    </select>


    <select id="getCitysByPid" resultMap="BaseResultMap">
        select 
        <include refid="Base_Column_List"/>
        from citys
        where pid=#{pid}

    </select>
 
</mapper>

直接调用：

@Test
    public void testGetCitys(){
        List<Citys> list = this.citysMapper.getCitys();
        list.forEach(System.out::println);
    }

打印结果：