In this tutorial, I am going to show you how to work with Hibernate Left Join.

What is Left Join?

The Left Join is a keyword in SQL, which returns all data from the left-hand side table and matching records from the right-hand side table. We can see the NULL values from the right side if there is no match.

Hibernate Left Join :

We can apply the Joins in Hibernate by using the HQL query or native SQL query. To make a join between the two tables, the two tables must be in a logical relationship. We can achieve the relationship between two tables by applying the parent table’s primary key as a child table’s foreign key.

Hibernate Left Join Example :

For this tutorial, I am going to take the two tables like Customer and Items; these two are having one to many relationship. That means one Customer can have multiple Items.

Project Structure :

Create customer and item tables with data to apply left join.

Customer Table: Create item Table: Customer.java

package com.otp.hibernate.pojo;

import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name = "customer")
public class Customer {
    @Id
    @Column(name = "cid")
    private int customerId;
    @Column(name = "cname", length = 15)
    private String customerName;
    @Column(name = "ccity", length = 20)
    private String customerCity;

    @OneToMany(targetEntity = Item.class, cascade = CascadeType.ALL)
    @JoinColumn(name = "cid",referencedColumnName="cid")
    private List items;

    public int getCustomerId() {
        return customerId;
    }

    public void setCustomerId(int customerId) {
        this.customerId = customerId;
    }

    public String getCustomerName() {
        return customerName;
    }

    public void setCustomerName(String customerName) {
        this.customerName = customerName;
    }

    public String getCustomerCity() {
        return customerCity;
    }

    public void setCustomerCity(String customerCity) {
        this.customerCity = customerCity;
    }

    public List getItems() {
        return items;
    }

    public void setItems(List items) {
        this.items = items;
    }

}

Create Item Class

package com.otp.hibernate.pojo;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="item")
public class Item {
    @Id
    @Column(name="iid")
    private int itemId;
    @Column(name="iname")
    private String itemName;
    @Column(name="price")
    private int price;

    public int getItemId() {
        return itemId;
    }

    public void setItemId(int itemId) {
        this.itemId = itemId;
    }

    public String getItemName() {
        return itemName;
    }

    public void setItemName(String itemName) {
        this.itemName = itemName;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }

}

Create Hibernate Configuration File :

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.springframework.samples</groupId>
  <artifactId>Hibernate-LeftJoin-Example</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <dependencies>
    <dependency>
      <groupId>org.hibernate</groupId>
      <artifactId>hibernate-core</artifactId>
      <version>4.3.0.Final</version>
    </dependency>
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>5.1.34</version>
    </dependency>
  </dependencies>
</project>

Create Hibernate Utility Class

package com.otp.hibernate.pojo;

import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class HibernateUtility{
    private static SessionFactory factory;

    private HibernateUtility() {
    }

    public synchronized static SessionFactory getSessionFactory() {
        if (factory == null) {
            factory = new Configuration().configure("hibernate.cfg.xml")
                    .buildSessionFactory();
        }
        return factory;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return new RuntimeException("Clone not Supported");
    }

}

Run the application :

Query For Hibernate Left Join on the above tables, the same will be implemented in below class as hql select query.

 select c.customerName, c.customerCity, i.itemName,i.price from Customer c left join c.items i;

Main.java

import java.util.Iterator;
import java.util.List;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;

import com.otp.hibernate.pojo.HibernateUtility;

public class Main {

    public static void main(String[] args) {
        SessionFactory factory = HibernateUtility.getSessionFactory();
        Session session = factory.openSession();
        Query qry= session.createQuery("select c.customerName, c.customerCity, i.itemName,i.price from Customer c "
                + "left join c.items i");
        List l = qry.list();
        Iterator it=l.iterator();
        while(it.hasNext())
        {
            Object rows[] = (Object[])it.next();
            System.out.println(rows[0]+ " -- " +rows[1] + "--"+rows[2]+"--"+rows[3]);
        }
        session.clear();
        session.close();
    }

}

Output:

Happy Learning 🙂