Java jpa外连接查询join案例详解

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1、IndexTagController.java

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@GetMapping("/tags/{id}")
    public String types(@PageableDefault(size = 3,sort = {"updateTime"},direction = Sort.Direction.DESC)Pageable pageable,
                        @PathVariable long id,
                        Model model,
                        HttpSession session){
        //找到所有的标签，并且按照标签新闻量排序
        List<Tag> tags = tagService.listTagTop(50);
        if(id == -1){
            //得到最大数据量的分类
            id = tags.get(0).getId();
        }

        model.addAttribute("tags",tags);
        model.addAttribute("page",newsService.listNews(id,pageable));
        model.addAttribute("activeId",id);
        session.setAttribute("query","");
        return "tags";
    }

newService.listNews(id,pgeable)中id为标签的id，这个方法要做的就是查询出标签中包含id为参数id的所有新闻。

2、业务层代码

NewService.java是一个接口，其中存在以下方法

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//根据标签Id查找符合条件的新闻
Page<News> listNews(long id,Pageable pageable);

NewServiceImpl.java为实现NewService接口的类，实现listNews方法

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@Override
    public Page<News> listNews(long id, Pageable pageable) {
        return newsRepository.findAll(new Specification() {
            @Override
            public Predicate toPredicate(Root root, CriteriaQuery cq, CriteriaBuilder cb) {
                //外连接查询 Join
                Join join =root.join("tags");
                return cb.equal(join.get("id"),id);
            }
        },pageable);

    }

NewsRepository.java 继承了JpaSpecificationExecutor

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public interface NewsRepository extends JpaRepository<News,Long>, JpaSpecificationExecutor {

    @Query("select n from News n where n.recommend = true ")
    List<News> findTop(Pageable pageable);


    @Query("select n from News n where n.title like ?1 or n.content like ?1")
    Page<News> findByQuery(String query,Pageable pageable);

    @Query("select function('date_format',n.updateTime,'%Y') as year1 from News n group by year1 order by year1 desc ")
    List<String> findGroupYear();

    @Query("select n from News n where function('date_format',n.updateTime,'%Y') = ?1 ")
    List<News> findByYear(String year);

}