java中的数据库查询流程

controller

@RequestMapping(value = "/list")
@ResponseBody
@ApiOperation("社区列表")
public ApiResponse<List<CommunityZone>> list() {
    return ApiResponseUtils.success(zoneService.getAll());
}

service

List<CommunityZone> getAll();

ServiceImpl

@Override
public List<CommunityZone> getAll() {
    List<CommunityZone> list = communityMapper.allAndVillageNum();
    return list;
}

Mapper

package com.smart.mapper.userExtendInfo;

import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.smart.model.userExtendInfo.Community;
import com.smart.model.userExtendInfo.pojo.CommunityZone;

import java.util.List;


/**
 * <p>
 * 社区基础信息表 Mapper 接口
 * </p>
 */
public interface CommunityMapper extends BaseMapper<Community> {
    /**
     * 社区列表
     * @return
     */
    List<CommunityZone> allAndVillageNum();
    Integer count();
}

pojo，model

package com.smart.model.userExtendInfo.pojo;
import com.smart.model.userExtendInfo.Community;
import io.swagger.annotations.Api;
import io.swagger.annotations.ApiModelProperty;

@Api("社区查询结果集")
public class CommunityZone extends Community {
    @ApiModelProperty("下辖小区数量")
    private Integer villiageNumbers;
    public Integer getVilliageNumbers() {
        return villiageNumbers;
    }
    public void setVilliageNumbers(Integer villiageNumbers) {
        this.villiageNumbers = villiageNumbers;
    }
}

package com.smart.model.userExtendInfo;

import com.baomidou.mybatisplus.annotation.IdType;
import com.baomidou.mybatisplus.annotation.TableId;
import com.fasterxml.jackson.annotation.JsonFormat;
import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.Getter;
import lombok.Setter;
import lombok.experimental.Accessors;

import java.io.Serializable;
import java.util.Date;
import java.util.Date;

/**
 * <p>
 * 社区基础信息表
 * </p>
 */
@Data
@EqualsAndHashCode(callSuper = false)
@Getter
@Setter
@Accessors(chain = true)
public class Community implements Serializable {

    private static final long serialVersionUID = 1L;

    @TableId(value = "id", type = IdType.AUTO)
    private Integer id;

    private String name;

    private Date createTime;

    private Integer creatorId;

    /**
     * 范围
     */
    private String limits;

    private String address;

    private String phone;

    public static long getSerialVersionUID() {
        return serialVersionUID;
    }

}

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.smart.mapper.userExtendInfo.CommunityMapper">
    <select id="allAndVillageNum" resultType="com.smart.model.userExtendInfo.pojo.CommunityZone">
       SELECT a.*,count(b.id) villiage_numbers from community a
       LEFT JOIN village b on a.id=b.community_id
       GROUP BY a.id
    </select>
</mapper>

逐层向下，环环相扣。