spring - 如何使用JdbcTemplate执行Join查询

Question

我有以下数据库模型

       create table Diary (id bigint NOT NULL AUTO_INCREMENT,
                creationDate TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
                name varchar(255) not null, 
                description text,
                viewtype varchar(255) not null,
                member bigint,
                primary key (id),
                foreign key (member) references Member(id));


       create table Page (id bigint NOT NULL AUTO_INCREMENT,
                creationDate TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
                viewtype varchar(255) not null,
                diary bigint,
                member bigint,
                primary key (id),
                foreign key (diary) references Diary(id),
                foreign key (member) references Member(id));

       create table Comment (id bigint NOT NULL AUTO_INCREMENT,
                postingDate TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
                comment text not null,
                page bigint,
                member bigint,
                primary key (id),
                foreign key (page) references Page(id)
                foreign key (member) references Member(id));

我正在使用 Spring JDBC 模板。

      My interface looks like follows: accountid is the memeberid in the database.

     Collection<Diary> getDiaries(Long accountId);

我的日记如下所示:

           public class Diary {
           private Collection<Page> pages;
           private Long id;
           private LocalTime creationDate;
           private String name;
           private String description;
           private ViewType type;
           }

如果我想使用 jdbc 模板准备日记对象，我想知道查询现在会是什么样子。也可以只触发一个查询并准备这个 Diary 对象，因为我将避免同一请求多次触发查询。对于上面的接口(interface)，我很可能会使用 s join 查询，或者是否有使用 spring JDBC 模板框架的更简单的方法。

Answer 1

您将能够使用外连接创建单个查询(我在这里假设日记可能没有任何页面并且页面没有评论)。

现在，不再执行多个查询(每页一个)，而是使用外部联接来连接 Diary、Page 和 Comment。如下所示，这意味着日记和页面信息会被多次返回，但我认为在多次数据库调用和一些冗余信息之间存在权衡。

 class FullDiaryRowCallbackHandler implements RowCallbackHandler {
    private Collection<Diary> diaries = new ArrayList<Diary>();
    private Diary currentDiary = null;
    private Page currentPage = null;

    public void processRow(ResultSet rs) {
       long diaryId = rs.getLong("d.id");
       if (currentDiary == null || diaryId != currentDiary.getId()) {
          currentDiary = new Diary();
          currentPage = null;
          diaries.add(currentDiary);
          currentDiary.setId(diaryId);
          currentDiary.setCreationDate(toLocalTime(rs.getTimestamp("d.creationDate")));
          currentDiary.setDescription(rs.getString("d.description"));
          ...
       }
       long pageId = rs.getLong("p.id");
       if (!rs.wasNull() && currentPage != null && currentPage.getId() != pageId) {
          currentPage = new Page();
          if (currentDiary.getPages() == null) {
              currentDiary.setPages(new ArrayList<Page>());
          }
          currentDiary.getPages().add(currentPage);
          currentPage.setId(pageId);
          currentPage.setCreationDate(toLocalTime(rs.getTimestamp("p.creationDate")));
          ...
       }
       long commentId = rs.getLong("c.id");
       if (!rs.wasNull() && currentPage != null) {
          Comment comment = new Comment();
          if (currentPage.getComments() == null) {
              currentPage.setComments(new ArrayList<Comment>());
          }
          currentPage.getComments().add(comment);
          comment.setId(commentId);
          comment.setPostingDate(toLocalTime(rs.getTimestamp("c.postingDate")));
          comment.setComment(rs.getString("c.comment"));
       }
    }

    public Collection<Diary> getDiaries() {
       return diaries;
    }
 }

 FullDiaryRowCallbackHandler rowCallbackHandler = new FullDiaryRowCallbackHandler();
 Collection<Diary> result = jdbcTemplate.query(
    "select d.id, " +
           "d.creationDate, " +
           "d.description, " +
           "p.id, " +
           "p.creationDate, " +
           "c.id, " +
           "c.postingDate, " +
           "c.comment " +
      "from Diary d " +
      "left outer join Page p on d.id = p.diary " +
      "left outer join Comment c on p.id = c.page " +
     "where d.member = ? " +
     "order by d.id, p.id, c.id",
    rowCallbackHandler,
    myMemberId);
Collection<Diary> diariesForMember = rowCallbackHandler.getDiaries();

请注意，代码不是特别漂亮，因为您必须处理结果集并注意返回新页面的时间(这就是 order by 子句很重要的原因)，但是这就是 Hibernate 之类的东西在急切地获取数据时会在幕后为您提供帮助(我并不是说 Hibernate 更好或不好，因为我喜欢使用 JdbcTemplate 来控制它对我的查询提供的控制)我发布了，但是 Hibernate(或 JPA)在填充对象图时做了很多繁重的工作)。

另请参阅JdbcTemplate#query

编辑:

修改为返回成员(member)的所有日记、页面、评论。