如何在hibernate中使用子查询到"from"子句？

Question

我有三个表:class,student,和teacher

table class
{
    class_id(PK)
}

table student
{
    student_id(PK)
    class_id(PK+FK)
}

table teacher
{
    teacher_id(PK)
    class_id(PK+FK)
}

我在SQL中有一个查询,工作正常.

SELECT data.class_id, count(data.class_id) AS count
FROM ((SELECT class_id FROM student)
        union all
        (SELECT class_id FROM teacher)) AS data
GROUP BY data.user_id
ORDER BY count desc

该查询包含from子句和union操作中的子查询.我无法将其转换为HQL.

请从上面的SQL查询中给我高效的HQL查询.

Answer 1

不幸的是,HQL不支持UNION查询.解决问题的两种替代策略是:

• 将Pojo映射到视图

• 或继承映射.特别是每个具有抽象Person超类的具体类策略的表继承了Student并且Teacher似乎很适合您的问题:

  select p.id, count(c)
  from Person p join p.classes c
  group by p.id
  order by count(c) desc
  

Answer 2

您不能from在HQL 中的子句中放置子查询.只有select或where条款.