java - Hibernate 标准 aPI 多选

Question

如果我使用 Hibernate Criteria API，例如:

CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
CriteriaQuery criteriaQuery = criteriaBuilder.createQuery();
Root<OneEntity> entityOneRoot = criteriaQuery.from(OneEntity.class);
Root<TwoEntity> entityTwoRoot = criteriaQuery.from(TwoEntity.class);
criteriaQuery.multiselect(OneEntity, TwoEntity);

我是否需要使用 EQUAL 限制(如 SQL 中的 ON 限制)来使两个表(实体)之间的 ID 相等？因为两个表的笛卡尔积？

criteriaQuery.where(criteriaBuilder.equal(OneEntity.get("fk_id"), TwoEntity.get("id")));

我的意思是...在使用 join 的 SQL 中，我们需要使用 on-clausule，例如:

select * from table_1 t1, table_2 t2 where t1.t2_id=t2.t1_id;

但我在 API Criteria 中找不到有关它的信息。

Answer 1

来自 JPA 2.1 规范，章节 4.4.5 连接:

An inner join may be implicitly specified by the use of a cartesian product in the FROM clause and a join condition in the WHERE clause. In the absence of a join condition, this reduces to the cartesian product.

The main use case for this generalized style of join is when a join condition does not involve a foreign key relationship that is mapped to an entity relationship.

Example:

SELECT c FROM Customer c, Employee e WHERE c.hatsize = e.shoesize

In general, use of this style of inner join (also referred to as theta-join) is less typical than explicitly defined joins over relationships.

由于 JPQL 的工作方式与 Criteria API 相同，因此您的查询中需要同样的限制。