Java jpa外连接查询join案例详解

@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" ;

     }

//根据标签Id查找符合条件的新闻

Page<News> listNews( long id,Pageable pageable);

@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);

     }

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);

}