Transforming Native SQL to JPQL: Leveraging CTEs and `@SqlResultSetMapping`

Is it possible to transform a query joining onto a subselect into JPQL?

Given the following native SQL query containing a join to a subselect, is there a way to transform it into a JPQL query (or alternatively, is it possible to map this using <code>@SqlResultSetMapping</code> such that I don’t have to execute thousands of subsequent queries to populate my objects?

SELECT 
    foo.*, bar.*, baz.*
FROM
    foo
        INNER JOIN
foo.bar ON foo.bar_id = bar.id
        INNER JOIN
bar.baz ON bar.baz_id = baz.id
        INNER JOIN
(
    SELECT 
        bar_id, MAX(some_int) ct
    FROM
        foo
    WHERE
        some_int <= 2
    GROUP BY bar_id) max_id ON max_id.bar_id = foo.bar_id
WHERE
    foo.some_int = max_id.ct;

This native SQL query is complex and contains several joins. The question asks whether it’s possible to transform this query into a JPQL query.

Native SQL vs JPQL

Before we dive into the details, let’s briefly discuss the differences between native SQL and JPQL (Java Persistence Query Language).

Native SQL is used to interact directly with the database. It’s a standard language for querying relational databases. On the other hand, JPQL is a Java-based query language that allows us to define queries in a more object-oriented way.

Limitations of JPQL

While JPQL offers many benefits, such as being a Java-based language and providing better support for complex queries, it also has some limitations. One of these limitations is the inability to directly execute subqueries in the FROM clause.

In native SQL, we can easily join onto a subselect using the following syntax:

SELECT 
    foo.*, bar.*, baz.*
FROM
    foo
        INNER JOIN
foo.bar ON foo.bar_id = bar.id
        INNER JOIN
bar.baz ON bar.baz_id = baz.id
        INNER JOIN
(
    SELECT 
        bar_id, MAX(some_int) ct
    FROM
        foo
    WHERE
        some_int <= 2
    GROUP BY bar_id) max_id ON max_id.bar_id = foo.bar_id
WHERE
    foo.some_int = max_id.ct;

However, in JPQL, this is not directly possible. We need to use a different approach.

Using CTEs and `@SqlResultSetMapping`

One way to achieve this is by using Common Table Expressions (CTEs) and the <code>@SqlResultSetMapping</code> annotation.

Let’s first define an entity class that represents our query:

@Entity
public class MyMaxEntity {
  @Id Integer id;
  Integer someInt;
}

Next, we’ll create a CTE that will serve as our subquery:

@CTE
public class MyMaxEntity {
  @Id Integer id;
  Integer someInt;
}

Now, let’s define the JPQL query using this CTE:

criteriaBuilderFactory.create(entityManager, Foo.class, "foo")
  .fetch("bar.baz")
  .innerJoinOnSubquery(MyMaxEntity.class, "maxEnt")
    .from(Foo.class, "subFoo")
    .bind("id").select("subFoo.id")
    .bind("value").select("MAX(subFoo.someInt)")
    .where("subFoo.someInt").le(2)
  .end()
  .on("maxEnt.id").eqExpression("foo.bar.id")
  .end()
  .where("foo.someInt").eqExpression("maxEnt.value")

In this query, we’re using the innerJoinOnSubquery method to join onto our CTE. We specify the MyMaxEntity class as our subquery and give it a name ("maxEnt").

The rest of the query is similar to the native SQL query we started with.

Benefits and Drawbacks

Using CTEs and <code>@SqlResultSetMapping</code> provides several benefits:

We can transform complex queries into JPQL.
We don’t have to execute thousands of subsequent queries to populate our objects.

However, there are also some drawbacks:

This approach requires the use of advanced features like CTEs and @SqlResultSetMapping.
It may not be suitable for all types of queries or applications.

Conclusion

In conclusion, while native SQL provides a more straightforward way to execute complex queries, JPQL offers more flexibility and power. By using CTEs and <code>@SqlResultSetMapping</code>, we can transform our native SQL query into a JPQL query that meets our needs.

This approach requires some upfront planning and configuration, but it ultimately provides better performance and maintainability.

Additional Resources

Last modified on 2023-09-01