​ 之前写代码一直用for循环读写数据库操作，直到在某个公众号看到一篇每日离职小技巧的文章，才知道问题的严重性。”读写数据库操作，写在for循环中，什么子查询，关联查询，左连接，右连接，通通不用，就是要先查一张表，然后遍历结果，再查另一张表…”。想想这不就是我嘛？这才明白当读取上万数据的时候，服务器内存是非常容易爆掉的，吓的我连夜学习了多表关联查询，，最终写了mapper.xml里的一对多查询如下：

  <resultMap id="NoteVO" type="com.easy.domain.NoteVO">
    <result property="id" column="n_id"/>
    <result property="nickName" column="nick_name"/>
    <result property="openid" column="n_openid"/>
    <result property="avatarUrl" column="avatar_url"/>
    <result property="desc" column="desc"/>
    <result property="createTime" column="create_time"/>
    <collection notNullColumn="comment" property="commentsList" javaType="java.util.List" ofType="com.easy.domain.NoteComments">
        <result property="noteId" column="n_id"/>
        <result property="comment" column="comment"/>
        <result property="openid" column="c_openid"/>
        <result property="nickName" column="c_nick_name"/>
    </collection>
    <collection property="noteLikes" javaType="java.util.List" ofType="com.easy.domain.NoteLike">
      <result property="noteId" column="l_n_id"/>
      <result property="id" column="l_id"/>
      <result property="openid" column="l_openid"/>
      <result property="nickName" column="l_nick_name"/>
    </collection>
    <collection property="noteImgs" javaType="java.util.List" ofType="com.easy.domain.NoteImgs">
      <result property="noteId" column="im_n_id"/>
      <result property="imgUrl" column="img_url"/>
    </collection>
  </resultMap>

  <select id="getAllList" resultMap="NoteVO" parameterType="int">
    SELECT note.id n_id,note.openid n_openid,note.`desc`,note.create_time,note.`type`,
           note_comments.note_id c_n_id,note_comments.`comment`,note_comments.openid c_openid,note_comments.nick_name c_nick_name,
           note_like.id l_id,note_like.note_id l_n_id,note_like.openid l_openid,note_like.nick_name l_nick_name,
           note_imgs.note_id im_n_id,note_imgs.img_url,
           `user`.openid,`user`.avatar_url,`user`.nick_name
    FROM note
                 LEFT OUTER JOIN note_comments ON note.id=note_comments.note_id
                 LEFT OUTER JOIN note_like ON note.id= note_like.note_id
                 LEFT OUTER JOIN note_imgs ON note.id=note_imgs.note_id
                 LEFT OUTER JOIN `user` ON  note.openid=`user`.openid
    WHERE note.type=#{type}
    ORDER BY note.create_time DESC
  </select>

再改了java代码：

 @Override
    public CommonResult getAllList(int type) {
        List<NoteVO> allList = noteMapper.getAllList(type);
        return CommonResult.success(allList);
    }

对比之前写的代码：

  @Override
    public CommonResult<List<NoteVO>> getAll(int type) {
        List<Note> notes = lambdaQuery().eq(Note::getType,type).orderByDesc(Note::getCreateTime).list();
//        List<Note> notes = noteMapper.selectList(new QueryWrapper<>());
        List<NoteVO> noteVOList = new ArrayList<>();
        for (Note note : notes) {
            Long noteId = note.getId();
            Date createTime = note.getCreateTime();
            String openid = note.getOpenid();
            String desc = note.getDesc();
            User user = userService.lambdaQuery().eq(User::getOpenid, openid).one();
            List<NoteLike> likeList = noteLikeService.lambdaQuery().eq(NoteLike::getNoteId, noteId).list();
            List<NoteImgs> imgsList = noteImgsService.lambdaQuery().eq(NoteImgs::getNoteId, noteId).list();
            List<NoteComments> commentsList = noteCommentsService.lambdaQuery().eq(NoteComments::getNoteId, noteId).list();
            NoteVO noteVO = new NoteVO();
            String nickName = user.getNickName();
            String avatarUrl = user.getAvatarUrl();
            //System.out.println(user);
            noteVO.setId(noteId);
            noteVO.setOpenid(openid);
            noteVO.setAvatarUrl(avatarUrl);
            noteVO.setCommentsList(commentsList);
            noteVO.setCreateTime(createTime);
            noteVO.setDesc(desc);
            noteVO.setNoteImgs(imgsList);
            noteVO.setNoteLikes(likeList);
            noteVO.setNickName(nickName);
            noteVOList.add(noteVO);
        }
        return CommonResult.success(noteVOList);
    }

查询的效率大大提高。

还是需要继续学习啊。。