Java.Hibernate.Medium.How do you join tables using HQL?

In JPA, you join tables by navigating entity relationships in JPQL/HQL queries, using JOIN, LEFT JOIN, or JOIN FETCH — just like in Hibernate HQL — since JPA uses JPQL as its standard query language.

🔎 Detailed Explanation

JPA Joins use mapped entity associations, not raw SQL joins on tables.
You specify joins on entity fields annotated with relationships like @ManyToOne, @OneToMany, etc.

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🔹 Basic Inner Join Example
Given entities:

@Entity
public class Order {
    @ManyToOne
    private Customer customer;
}

JPQL join:

String jpql = "SELECT o FROM Order o JOIN o.customer c WHERE c.status = :status";
List<Order> orders = entityManager.createQuery(jpql, Order.class)
                                  .setParameter("status", "ACTIVE")
                                  .getResultList();

Here, o.customer navigates the Order→Customer relationship.

🔹 Left Outer Join Example
Loads all orders even if they don’t have a customer:

String jpql = "SELECT o FROM Order o LEFT JOIN o.customer c";
List<Order> orders = entityManager.createQuery(jpql, Order.class)
                                  .getResultList();

🔹 Fetch Join Example
Eagerly loads customers along with orders in a single query (avoiding N+1 problem):

🔹 Fetch Join Example
Eagerly loads customers along with orders in a single query (avoiding N+1 problem):

🔹 Join with Multiple Levels
If you have deeper relationships, you can join multiple associations:

String jpql = "SELECT o FROM Order o " +
              "JOIN o.customer c " +
              "JOIN c.address a " +
              "WHERE a.city = :city";
List<Order> orders = entityManager.createQuery(jpql, Order.class)
                                  .setParameter("city", "New York")
                                  .getResultList();