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

查询包含 from子句和 union operation 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 does not support UNION queries。解决问题的两种替代策略是：

映射Pojo to a a view
或Inheritance mapping。特别是由Person和Student继承的抽象Teacher超类的Table per concrete class策略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

您不能在HQL中的from子句中放置子查询。仅限select或where条款。

如何在hibernate中使用子查询到“from”子句？

2 个答案: