How to create a JPA query with LEFT OUTER JOIN

问题:

I am starting to learn JPA, and have implemented an example with JPA query, based on the following native SQL that I tested in SQL Server:

SELECT f.StudentID, f.Name, f.Age, f.Class1, f.Class2 
FROM Student f 
    LEFT OUTER JOIN ClassTbl s ON s.ClassID = f.Class1 OR s.ClassID = f.Class2
WHERE s.ClassName = 'abc'

From the above SQL I have constructed the following JPQL query:

SELECT f FROM Student f LEFT JOIN f.Class1 s;

As you can see, I still lack the condition OR s.ClassID = f.Class2 from my original query. My question is, how can I put it into my JPQL?

回答1:

Write this;

 SELECT f from Student f LEFT JOIN f.classTbls s WHERE s.ClassName = 'abc'

Because your Student entity has One To Many relationship with ClassTbl entity.

回答2:

If you have entities A and B without any relation between them and there is strictly 0 or 1 B for each A, you could do:

select a, (select b from B b where b.joinProperty = a.joinProperty) from A a

This would give you an Object[]{a,b} for a single result or List<Object[]{a,b}> for multiple results.

回答3:

Normally the ON clause comes from the mapping's join columns, but the JPA 2.1 draft allows for additional conditions in a new ON clause.

See,

http://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Querying/JPQL#ON