How to customize the results of a JPA query using aggregate functions?

Most of the time, when we use JPA queries, the results obtained are mapped to objects/specific data types. But when we use aggregate functions in queries, processing the results sometimes requires us to customize the JPA query.

Let us understand (department, employee) through an example −

Dept.java

@Entity
public class Dept {
   @Id
   private Long id;
   private String name;
   @OneToMany(mappedBy = "dep")
   private List<Employee> emp;
   //Getters
   //Setters
}

A department can have one or more employees, but an employee can only belong to one department.

Employee.java

@Entity
public class Employee {
   @Id
   private Long id;
   private Integer joiningyear;
   @ManyToOne
   private Dept dep;
   
   //Getters
   //Setters
}

Now, if we want to get the joining date and the number of employees grouped by joining date,

@Repository
public interface EmployeeRepository extends JpaRepository<Employee, Long> {
   // query methods
   @Query("SELECT e.joiningyear, COUNT(e.joiningyear) FROM Employee AS e GROUP BY e.joiningyear")
   List<Object[]> countEmployeesByJoiningYear();
}

The above query works fine, but storing values ​​in the form of List may cause errors. Instead, we can customize the JPA query to map the results of the above query into a Java class. This Java class is just a simple POJO (Plain Old Java Object) and does not need to be annotated with @Entity.

CountEmployees.java

CountEmployees.java

package com.tutorialspoint;
public class CountEmployees {
   private Integer joinyear;
   private Long totalEmp;
   
   public CountEmployees(Integer joinyear, Long totalEmp) {
      this.joinyear = joinyear;
      this.totalEmp = totalEmp;
   }
   //Getters
   //Setters
}

Now, we can customize our JPA query as shown below −

@Query("SELECT new com.tutorialspoint.CountEmployees(e.joiningyear, COUNT(e.joiningyear)) " + "FROM Employee AS e GROUP BY e.joiningyear")
List<CountEmployees> countEmployeesByJoining();

The results of the above select query will be mapped to the CountEmployees class. In this way we can customize JPA queries and map the results to java classes.

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