Question

我有一个非实体类

public class CountryStatistics {

    public CountryStatistics(Long numTowns, Long numVillages) {
    ...
    }

}

对于给定的国家/地区，我想构建一个统计对象。

国家，村庄和城市是实体，所以我尝试了下面的代码。

String queryString = 
"SELECT NEW mypackage.CountryStatistics(count(t), count(v)) 
 FROM Town t, Village v WHERE t.country = :country AND v.country = :country"

TypedQuery<CountryStatistics> query = 
em.createQuery(queryString ,CountryStatistics.class).setParameter("country", country);

query.getSingleResult()

问题：在同一查询中计算不同表中某些实体的正确方法是什么？

通过上面的查询，如果我像下面那样区分，我最终将获得高数字，城镇数量将是正确的。

"SELECT NEW mypackage.CountryStatistics(count(distinct t), count(v)) 
 FROM Town t, Village v WHERE t.country = :country AND v.country = :country"

但如果我把它设置为村庄，我也会得到：

java.sql.SQLSyntaxErrorException: Multiple DISTINCT aggregates are not supported at this time.

Answer 1

以下查询（未经测试）应该执行您想要的操作：

select count(distinct t.id), count(distinct v.id) from Country c
left join c.towns t
left join c.villages v
where c.id = :countryId

计算多个表中的实体

1 个答案: