如何使用LEFT OUTER JOIN创建JPA查询？

Question

我正在学习JPA，所以我想使用这样的mysql查询

SELECT t1.name AS lev1, t2.name as lev2, t3.name as lev3, t4.name as lev4 
FROM category AS t1 LEFT 
JOIN category AS t2 ON t2.parent = t1.category_id 
LEFT JOIN category AS t3 ON t3.parent = t2.category_id 
LEFT JOIN category AS t4 ON t4.parent = t3.category_id
WHERE t1.name = 'ROOT'

那么如何将其更改为jpql查询代码？谢谢

Answer 1

如果您使用的是EntityManager，则可以按照这种方式进行操作

@PersistenceContext private EntityManager entityManager;

StringBuilder sb = new StringBuilder();

sb.append("SELECT t1.name AS lev1, t2.name as lev2, t3.name as lev3, t4.name as lev4 
FROM category AS t1 LEFT 
JOIN category AS t2 ON t2.parent = t1.category_id 
LEFT JOIN category AS t3 ON t3.parent = t2.category_id 
LEFT JOIN category AS t4 ON t4.parent = t3.category_id
WHERE t1.name = :name"); // :name - you can pass parameter here 

Query q = entityManager.createQuery(sb.toString());


 q.setParameter("name", "ROOT");//Root is the value and name is the parameter 

Answer 2

如果使用的是Spring数据jpa，则可以在存储库层中提供它，如：

@Repository
    public interface CategoryRepository extends JpaRepository<Category, Integer> {
        @Query(
    "SELECT t1.name AS lev1, t2.name as lev2, t3.name as lev3, t4.name as lev4 
    FROM category AS t1" +
    "LEFT JOIN category AS t2 ON t2.parent = t1.category_id" +
    "LEFT JOIN category AS t3 ON t3.parent = t2.category_id" + 
    "LEFT JOIN category AS t4 ON t4.parent = t3.category_id" +
    "WHERE t1.name = 'ROOT'")
        List<DeptEmpDto> fetchCategoryDataLeftJoin();

    }