Mapper.XML

<!--联合查询用到的map-->
    <resultMap type="LmbBirdCollect" id="LmbBirdCollectResult2">
        <id property="collectId"    column="collect_id"    />
        <result property="pointId"    column="point_id"    />
        <result property="collectTime"    column="collect_time"    />
        <result property="status"    column="status"    />
        <result property="delFlag"    column="del_flag"    />
        <result property="createBy"    column="create_by"    />
        <result property="createTime"    column="create_time"    />
        <result property="updateBy"    column="update_by"    />
        <result property="updateTime"    column="update_time"    />
        <result property="remark"    column="remark"    />
        <!--一对一，使用association,property是在LmbBirdCollect实例类中的字段名，column是在LmbBirdCollect中和LmbBirdPoint相关联的字段-->
        <association property="birdPoint" column="point_id" javaType="LmbBirdPoint" >
            <id property="pointId"    column="point_id"    />
            <result property="itemId"    column="item_id"    />
            <result property="pointName"    column="point_name"    />
            <result property="longitude"    column="longitude"    />
            <result property="latitude"    column="latitude"    />
            <result property="altitude"    column="altitude"    />
            <result property="remarks"    column="remarks"    />
            <result property="status"    column="status"    />
        </association>
        <!--一对多，使用collection，property是LmbBirdCollect实例类中的字段名，column是在LmbBirdCollect实例类和LmbBirdCollects关联的字段，注意：这里要使用ofType-->
        <collection property="birdCollects" column="collect_id" ofType="LmbBirdCollects">
            <id property="collecstId"    column="collects_id"    />
            <result property="collectId"    column="collect_id"    />
            <result property="birdId"    column="bird_id"    />
            <result property="quantity"    column="quantity"    />
        </collection>
    </resultMap>

<select id="selectLmbBirdCollectByCollectId" parameterType="Long" resultMap="LmbBirdCollectResult2">
        SELECT c.collect_id, c.point_id, c.collect_time, c.status, c.del_flag, c.create_by,
               c.create_time, c.update_by, c.update_time, c.remark ,p.point_name,s.bird_id,s.quantity
        FROM lmb_bird_collect c
        LEFT JOIN lmb_bird_point p ON c.point_id=p.point_id
        LEFT JOIN lmb_bird_collects s ON c.collect_id=s.collect_id
        where c.collect_id = #{collectId}
 </select>    

实体类：

public class LmbBirdCollect extends BaseEntity
{
    private static final long serialVersionUID = 1L;

    /** 自增id */
    private Long collectId;

    /** 所属区域 */
    @Excel(name = "所属点")
    private Long pointId;

    /** 创建时间 */
    @Excel(name = "调查时间", width = 30, dateFormat = "yyyy-MM-dd")
    @JsonFormat(pattern = "yyyy-MM-dd")
    private Date collectTime;

    /** 鸟种 */
    @Excel(name = "状态")
    private String status;

    private LmbBirdPoint birdPoint;
    private List<LmbBirdCollects> birdCollects;

    public List<LmbBirdCollects> getBirdCollects() {
        if(birdCollects == null){
            List<LmbBirdCollects> birdCollects = new ArrayList<>();
        }
        return birdCollects;
    }

    public void setBirdCollects(List<LmbBirdCollects> birdCollects) {
        this.birdCollects = birdCollects;
    }

    public LmbBirdPoint getBirdPoint() {
        if(birdPoint == null){
            birdPoint = new LmbBirdPoint();
        }

        return birdPoint;
    }

    public void setBirdPoint(LmbBirdPoint birdPoint) {
        this.birdPoint = birdPoint;
    }

    /** 删除标志(0代表存在 2代表删除) */
    private String delFlag;

    public String getDelFlag() {
        return delFlag;
    }

    public void setDelFlag(String delFlag) {
        this.delFlag = delFlag;
    }

    public Long getCollectId() {
        return collectId;
    }

    public void setCollectId(Long collectId) {
        this.collectId = collectId;
    }

    public Long getPointId() {
        return pointId;
    }

    public void setPointId(Long pointId) {
        this.pointId = pointId;
    }

    public Date getCollectTime() {
        return collectTime;
    }

    public void setCollectTime(Date collectTime) {
        this.collectTime = collectTime;
    }

    public String getStatus() {
        return status;
    }

    public void setStatus(String status) {
        this.status = status;
    }

    @Override
    public String toString() {
        return "LmbBirdCollect{" +
                "collectId=" + collectId +
                ", pointId=" + pointId +
                ", collectTime=" + collectTime +
                ", status='" + status + '\'' +
                ", birdPoint=" + birdPoint +
                ", birdCollects=" + birdCollects +
                ", delFlag='" + delFlag + '\'' +
                '}';
    }
}