Java|Java jpa外连接查询join案例详解 Javajpa外连接查询join案例详解

1、IndexTagController.java

@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 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

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

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

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

NewsRepository.java 继承了JpaSpecificationExecutor

public interface NewsRepository extends JpaRepository, JpaSpecificationExecutor {@Query("select n from News n where n.recommend = true ")List findTop(Pageable pageable); @Query("select n from News n where n.title like ?1 or n.content like ?1")Page 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 findGroupYear(); @Query("select n from News n where function('date_format',n.updateTime,'%Y') = ?1 ")List findByYear(String year); }

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