What is the procedure for executing an inner join query using Hibernate Panache?

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To execute an inner join query using Hibernate Panache, follow these steps:

1. Define the entities that you want to join using the standard JPA annotations.

2. Create a repository for each entity using the PanacheEntity base class.

3. Use the query builder provided by Hibernate Panache to define the join condition and the selection criteria for your query.

4. Call the find() method on the repository representing the outer entity to execute the query and retrieve the results.

For example, suppose you have two entities, Author and Book, with a one-to-many relationship between them. The Author entity has a collection of Book entities called books. To retrieve all authors and their associated books, you could use the following code:

AuthorRepository.java:

import io.quarkus.hibernate.orm.panache.PanacheRepository;

public interface AuthorRepository extends PanacheRepository<Author> {
}

BookRepository.java:

import io.quarkus.hibernate.orm.panache.PanacheRepository;

public interface BookRepository extends PanacheRepository<Book> {
}

Author.java:

import javax.persistence.*;
import java.util.*;

@Entity
@Table(name = "author")
public class Author {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String firstName;

    private String lastName;

    @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<Book> books = new HashSet<>();

    // getters and setters omitted for brevity
}

Book.java:

import javax.persistence.*;
import java.util.*;

@Entity
@Table(name = "book")
public class Book {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String title;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author_id")
    private Author author;

    // getters and setters omitted for brevity
}

To retrieve all authors and their associated books using an inner join, you could use the following code:

List<Author> authors = AuthorRepository.find("SELECT DISTINCT a FROM Author a " +
        "JOIN FETCH a.books b", Collections.emptyMap()).list();

This query uses the JPA join fetch syntax to perform an inner join between the Author and Book entities. It also uses the DISTINCT keyword to ensure that each Author entity is returned only once in the result set. Finally, it specifies the selection criteria using the FROM clause of the query.