在JPA 2.0中从没有外键的多个表中获取数据

Question

I've been stumbling upon followig issue for a couple of days now nad I can't make it to work. Here is the problem. I have four tables (A, B, C, D) which are not related to eachother via any kind of foreign key. Hovewer, they do have a column called, let's say, 'superId'.

The task is to take all the records from the A table, find records from the other ones with matching 'superId' (if they exist) and return them via JPA's constructor expression.

About JOINs. Since the tables have no relations, I can't do a left JOIN (or any other JOINs).

I tried to use MULTISELECT with a success, but it only works if I do an implicit joins with 'a.superId = b.superId'. This causes problems, since the three tables might not have matching records which will make the query to return empty set. This won't fly.

I have no other ideas, and this is crucial for my project to work. Please forgive me simple description of an issue - sending from my mobile.

Answer 1

You absolutely do not require the presence of a foreign key relationship to perform an arbitrary query in JPA2.

You can't "follow" a parent/child relationship, so you can't do your usual parentObject.childObject thing. You must instead use the Criteria API, or HQL, to construct a join.

See:

• Using the Criteria API to Create Queries
• Creating Queries Using the Java Persistence Query Language
• JPQL language reference: joins