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

✅ Short Answer

In HQL (and JPQL), you join tables by navigating entity associations using JOIN, LEFT JOIN, and their FETCH variants — using entity relationships, not table names.

🔎 Detailed Explanation

Types of joins you can use in HQL:
1️⃣ JOIN → inner join: returns only records with matching related entities.
2️⃣ LEFT JOIN → left outer join: returns all parent entities, even if related entity is null.
3️⃣ FETCH JOIN → eager loads related entities in a single query.

🔹 Inner Join Example
Imagine:

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

HQL join:

String hql = "SELECT o FROM Order o JOIN o.customer c WHERE c.name = :name";
List<Order> orders = session.createQuery(hql, Order.class)
                            .setParameter("name", "Alice")
                            .getResultList();

🔹 Left Join Example
Fetch orders with or without a customer:

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

🔹 Fetch Join Example
Fetch orders with customers loaded immediately (avoids lazy loading N+1 problem):

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

📊 Quick Comparison Table

Join Type	Returns	Use Case
JOIN	Only parent entities with related	Filtered data needing matching children
LEFT JOIN	All parent entities	Need parents even if no children exist
JOIN FETCH	Parent + child fully loaded	Avoid lazy loading (N+1 issue)

💡 Important Points

✅ HQL joins navigate entity relationships, not database table names or foreign keys directly.
✅ Joins happen via mapped fields (e.g., o.customer), not explicit ON clauses like raw SQL.
✅ JOIN FETCH is great to optimize performance when you know you’ll need related data.

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📌 Key Takeaways

✅ Use JOIN in HQL to connect related entities based on entity associations.
✅ Prefer JOIN FETCH for eager loading when you want to load associations in one query.
✅ HQL doesn’t use SQL-style ON clauses → joins happen through object navigation.