This example tries to make understand about the aggregate function of Hibernate with the help of example.

Hibernate Aggregate Functions(Associations and Joins)

This example tries to make understand about the aggregate function of Hibernate with the help of example. 
In Hibernate HQL queries are capable of returning the results of the aggregate functions on properties.  Collections can also be used with the aggregate functions with the select clause. Aggregate functions supports the following functions:

• min(...), max(...), sum(...), avg(...).


• count(*)


• count(distinct...), count(all..), count(...)

The distinct and all keywords used above are identical as in SQL. 
A unidirectional one-to-many association on a foreign key is rarely required.

<?xml version="1.0" encoding="UTF-8"?>  <!DOCTYPE hibernate-mapping  PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"  "http://hibernate.sourceforge.net /hibernate-mapping-3.0.dtd"> <hibernate-mapping>  <class name="net.roseindia.Dealer">   <id name="id" type="int">  <generator class="increment"/>   </id>   <property name="name" type="string"/>   <bag name="product" inverse="true"  cascade="all,delete-orphan">   <key column="did"/>   <one-to-many class="net.roseindia.Product"/>   </bag>  </class> </hibernate-mapping>

A unidirectional one-to-many association on a foreign key is rarely required.

<?xml version='1.0' encoding='utf-8'?> <!DOCTYPE hibernate-mapping  PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"  "http://hibernate.sourceforge.net/ hibernate-mapping-3.0.dtd"> <hibernate-mapping>  <class name="net.roseindia.Product">   <id name="id" type="int">  <generator class="increment"/>   </id>   <property name="name" type="string"/>   <property name="did" type="int"/>   <property name="price" type="double"/>   <many-to-one name="dealer" class=" net.roseindia.Dealer" column="did"  insert="false" update="false"/>  </class>   </hibernate-mapping>

In this example first we create the session object with the help of the SessionFactory interface. Then we use the createQuery() method of the Session object which returns a Query object. Now we use the openSession() method of the SessionFactory interface simply to instantiate the Session object. And the we retrieve the data from the database store it in that Query object and iterate this object with the help of Iterator and finally displays the requested data on the console.
Here is the hibernate code:

package net.roseindia; import java.util.Iterator; import java.util.List; import org.hibernate.Query; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.cfg.Configuration; public class AggregateFunctionExample {   /**  * @param args  */   public static void main(String[] args) {   // TODO Auto-generated method stub   Session sess = null;   try{   SessionFactory fact = new Configuration().configure().buildSessionFactory();   sess = fact.openSession();   String sql_query = "select d.name,p.name,sum(p.price) as  totalprice from Product p join p.dealer d group by p.name";   Query query = sess.createQuery(sql_query);   System.out.println("Dealer Name\t"+"Product Name\t"+"Price");   for(Iterator it= query.iterate();it.hasNext();){   Object[] row = (Object[]) it.next();   System.out.print(row[0]);   System.out.print("\t\t"+row[1]);   System.out.print("\t"+row[2]);   System.out.println();   }   sess.close();   }   catch(Exception e ){   System.out.println(e.getMessage());   }   } }

Output:

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).log4j:WARN Please initialize the log4j system properly.Hibernate: select dealer1_.name as col_0_0_, product0_.name as col_1_0_, sum(product0_.price) as col_2_0_ from Product product0_ inner join Dealer dealer1_ on product0_.did=dealer1_.id group by product0_.name Dealer Name  Product Name  Price Agrawal   Computer   23100.0 Ritu   HardDisk  2500.0 Agrawal   Keyboard   1500.0 Ritu  Laptop   200.0 Mohan   Mobile   15000.0 Mohan   PenDrive   200.0