Rewrite SQL query with JOINS in JPA - jpa

Question

I have this SQL query for MariaDB.
select #ref:=id as id, unique_id, reference_id
from mytable
join (select #ref:=id from mytable WHERE unique_id = 55544)tmp
where reference_id=#ref
https://www.db-fiddle.com/f/jKJodfVfvw65aMaVDyFySd/0
How this query can be implemented in HQL query? I would like to use it in JPA?

Answer 1

(Answer largely re-written after comments below)
JPA doesn't have built-in support for hierarchical queries. The main option is a native query.
E.g. with this entity class:
#Entity
public class MyTable {
#Id
#GeneratedValue
private int id;
private int uniqueId;
#ManyToOne
private MyTable reference;
// ... getters and setters ...
}
The following is an example of a native hierachical SQL query (actually against MySQL, just in case):
Query query = entityManager.createNativeQuery(
"select #ref\\:=id as id, unique_id, reference_id\r\n" +
"from my_table\r\n" +
"join (select #ref\\:=?)tmp\r\n" +
"where reference_id=#ref",
MyTable.class);
query.setParameter(1, 1);
query.getResultList();
This was chasing down a chain of references successfully.
(Other alternatives)
There probably aren't too many other options that can do this as a single query. If scalability is less of a concern, adding a back reference would be a simple way to navigate the model:
#OneToMany(mappedBy = "reference")
private Set<MyTable> backReferences;
Those would then be straightforward to recursively navigate. Clearly the relation defaults to lazy loading, so would add little overhead until used.

Answer 2

With #df778899's MyTable in spring-data it could look like:
#Repository
public interface MyRepository extends ...
#Query("select #ref:=id as id, unique_id, reference_id "+
"from mytable join (select #ref:=id from mytable WHERE unique_id = :pUid) tmp "+
"where reference_id=#ref", //just copy paste the query, use :pUid instead of constant...
nativeQuery = true) // and this!
List<MyTable> myCustomHirachicalQuery(#Param("pUid") Integer uid/*String/Long/...*/);
...