具有聚合内部连接的 JPA 条件查询

Question

我正在尝试编写一个 CriteriaQuery，它将查询每个城市的最新观察结果。城市由 city_code 字段定义，而最新记录由 observation_time 字段定义。

我可以很容易地用普通的方式写出来 SQL，但是我不明白如何用 jpa 标准来写 api。

select distinct m.* from 
 (select city_code cc, max(observation_time) mo
 from observations group by city_code) mx, observations m 
 where m.city_code = mx.cc and m.observation_time = mx.mo`

Answer 1

不幸的是，JPA 不支持 FROM 子句中的子查询。您需要编写本机查询或使用 FluentJPA.

之类的框架

Answer 2

当你打开效率低下时，这是可能的。因此，首先让我们将查询转换为逻辑等效查询：

select distinct m.* from observations m where 
m.observation_time = (select max(inn. observation_time) from observations inn 
                      where inn.city_code = m.city_code);

然后让我们将其转换为 JPA CriteriaQuery：

public List<Observation> maxForEveryWithSubquery() {
    CriteriaBuilder builder = entityManager.getCriteriaBuilder();
    CriteriaQuery<Observation> query = builder.createQuery(Observation.class);
    Root<Observation> observation = query.from(Observation.class);
    query.select(observation);

    Subquery<LocalDateTime> subQuery = query.subquery(LocalDateTime.class);
    Root<Observation> observationInner = subQuery.from(Observation.class);
    subQuery.where(
            builder.equal(
                    observation.get(Observation_.cityCode),
                    observationInner.get(Observation_.cityCode)
            )
    );
    Subquery<LocalDateTime> subSelect = subQuery.select(builder.greatest(observationInner.get(Observation_.observationTime)));
    query.where(
            builder.equal(subSelect.getSelection(), observation.get(Observation_.observationTime))
    );
    TypedQuery<Observation> typedQuery = entityManager.createQuery(query);
    return typedQuery.getResultList();
}

具有聚合内部连接的 JPA 条件查询

JPA Criteria query with inner join of aggregation

java

hibernate

jpa

criteria-api