JPA Tutorial - JPA Query AVG Simple Example

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The syntax for aggregate queries in JP QL is very similar to that of SQL.

There are five supported aggregate functions

• AVG
• COUNT
• MIN
• MAX
• SUM

The results may be grouped in the GROUP BY clause and filtered using the HAVING clause.

An aggregate JP QL query can use many of the aggregate functions within the same query:

SELECT d, COUNT(e), MAX(e.salary), AVG(e.salary)
FROM Department d JOIN d.employees e
GROUP BY d
HAVING COUNT(e) >= 5

AVG function in JPQL calculates the average for a property.

The following line calculates the average salary.

"SELECT AVG(e.salary) FROM Professor e")

Aggregate Queries

An aggregate query groups results and applies aggregate functions to obtain summary information about query results.

The syntax of an aggregate query is as follows:

 
SELECT <select_expression>
FROM <from_clause>
[WHERE <conditional_expression>]
[GROUP BY <group_by_clause>]
[HAVING <conditional_expression>]
[ORDER BY <order_by_clause>]

Example

The following code is from PersonDaoImpl.java.

package com.java2s.common;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

import org.springframework.transaction.annotation.Transactional;

@Transactional
public class PersonDaoImpl {
  public void test() {
    prepareData();
    
    List l = em.createQuery(
        "SELECT AVG(e.salary) FROM Professor e")
        .getResultList();
    for (Object p : l) {
      printResult(p);
    }
  }

  private void prepareData() {
    Professor p = new Professor();
    p.setId(0);
    p.setName("TOM");
    p.setSalary(1111L);

    Department d = new Department();
    d.setId(1);
    d.setName("Design");

    p.setDepartment(d);
    d.getProfessors().add(p);

    Phone phone = new Phone();
    phone.setId(1);
    phone.setNumber("111-111-1111");
    phone.setProfessor(p);
    
    em.persist(p);
    em.persist(phone);
    em.persist(d);
  }

  private static void printResult(Object result) {
    if (result == null) {
      System.out.print("NULL");
    } else if (result instanceof Object[]) {
      Object[] row = (Object[]) result;
      System.out.print("[");
      for (int i = 0; i < row.length; i++) {
        printResult(row[i]);
      }
      System.out.print("]");
    } else if (result instanceof Long || result instanceof Double
        || result instanceof String) {
      System.out.print(result.getClass().getName() + ": " + result);
    } else {
      System.out.print(result);
    }
    System.out.println();
  }

  @PersistenceContext
  private EntityManager em;
}

The following code is from Project.java.

package com.java2s.common;

import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.ManyToMany;

@Entity
@Inheritance
public class Project {
    @Id
    protected int id;
    protected String name;
    @ManyToMany
    protected Collection<Professor> employees = new ArrayList<Professor>();
    
    public int getId() {
        return id;
    }
    
    public void setId(int projectNo) {
        this.id = projectNo;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String projectName) {
        this.name = projectName;
    }
    
    public Collection<Professor> getProfessors() {
        return employees;
    }
    
    public void addProfessor(Professor employee) {
        if (!getProfessors().contains(employee)) {
            getProfessors().add(employee);
        }
        if (!employee.getProjects().contains(this)) {
            employee.getProjects().add(this);
        }
    }
    
    public String toString() {
        return getClass().getName().substring(getClass().getName().lastIndexOf('.')+1) + 
                " no: " + getId() + 
                ", name: " + getName();
    }
}

The following code is from Phone.java.

package com.java2s.common;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
public class Phone {
    @Id
    private long id;
    private String number;
    private String type;
    @ManyToOne
    Professor employee;
    
    public long getId() {
        return id;
    }
    
    public void setId(long id) {
        this.id = id;
    }
    
    public String getNumber() {
        return number;
    }
    
    public void setNumber(String phoneNo) {
        this.number = phoneNo;
    }
    
    public String getType() {
        return type;
    }
    
    public void setType(String phoneType) {
        this.type = phoneType;
    }
    
    public Professor getProfessor() {
        return employee;
    }

    public void setProfessor(Professor employee) {
        this.employee = employee;
    }

    public String toString() {
        return "Phone id: " + getId() + 
               ", no: " + getNumber() +
               ", type: " + getType();
    }
}

The following code is from Professor.java.

package com.java2s.common;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

@Entity
public class Professor {
    @Id
    private int id;
    private String name;
    private long salary;
    @Temporal(TemporalType.DATE)
    private Date startDate;
    
    @OneToOne
    private Address address;
    
    @OneToMany(mappedBy="employee")
    private Collection<Phone> phones = new ArrayList<Phone>();
    
    @ManyToOne
    private Department department;
    
    @ManyToOne
    private Professor manager;
    
    @OneToMany(mappedBy="manager")
    private Collection<Professor> directs = new ArrayList<Professor>();
    
    @ManyToMany(mappedBy="employees")
    private Collection<Project> projects = new ArrayList<Project>();

    public int getId() {
        return id;
    }
    
    public void setId(int empNo) {
        this.id = empNo;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }

    public long getSalary() {
        return salary;
    }

    public void setSalary(long salary) {
        this.salary = salary;
    }

    public Date getStartDate() {
        return startDate;
    }

    public void setStartDate(Date startDate) {
        this.startDate = startDate;
    }
    
    public Collection<Phone> getPhones() {
        return phones;
    }
    
    public void addPhone(Phone phone) {
        if (!getPhones().contains(phone)) {
            getPhones().add(phone);
            if (phone.getProfessor() != null) {
                phone.getProfessor().getPhones().remove(phone);
            }
            phone.setProfessor(this);
        }
    }
    
    public Department getDepartment() {
        return department;
    }
    
    public void setDepartment(Department department) {
        if (this.department != null) {
            this.department.getProfessors().remove(this);
        }
        this.department = department;
        this.department.getProfessors().add(this);
    }
    
    public Collection<Professor> getDirects() {
        return directs;
    }
    
    public void addDirect(Professor employee) {
        if (!getDirects().contains(employee)) {
            getDirects().add(employee);
            if (employee.getManager() != null) {
                employee.getManager().getDirects().remove(employee);
            }
            employee.setManager(this);
        }
    }
    
    public Professor getManager() {
        return manager;
    }
    
    public void setManager(Professor manager) {
        this.manager = manager;
    }

    public Collection<Project> getProjects() {
        return projects;
    }
    
    public void addProject(Project project) {
        if (!getProjects().contains(project)) {
            getProjects().add(project);
        }
        if (!project.getProfessors().contains(this)) {
            project.getProfessors().add(this);
        }
    }
    
    public Address getAddress() {
        return address;
    }
    
    public void setAddress(Address address) {
        this.address = address; 
    }
    
    public String toString() {
        return "Professor " + getId() + 
               ": name: " + getName() +
               ", salary: " + getSalary() +
               ", phones: " + getPhones() +
               ", managerNo: " + ((getManager() == null) ? null : getManager().getId()) +
               ", deptNo: " + ((getDepartment() == null) ? null : getDepartment().getId());
    }

}

The following code is from Address.java.

package com.java2s.common;

import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class Address {
    @Id
    private int id;
    private String street;
    private String city;
    private String state;
    private String zip;
    
    public int getId() {
        return id;
    }
    
    public void setId(int id) {
        this.id = id;
    }
    
    public String getStreet() {
        return street;
    }
    
    public void setStreet(String address) {
        this.street = address;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public String getState() {
        return state;
    }

    public void setState(String state) {
        this.state = state;
    }

    public String getZip() {
        return zip;
    }

    public void setZip(String zip) {
        this.zip = zip;
    }
    public String toString() {
        return "Address id: " + getId() + 
               ", street: " + getStreet() +
               ", city: " + getCity() +
               ", state: " + getState() +
               ", zip: " + getZip();
    }

}

The following code is from Department.java.

package com.java2s.common;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class Department {
    @Id
    private int id;
    private String name;
    @OneToMany(mappedBy="department")
    private Set<Professor> employees = new HashSet<Professor>();

    public int getId() {
        return id;
    }
    
    public void setId(int deptNo) {
        this.id = deptNo;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String deptName) {
        this.name = deptName;
    }
    
    public Set<Professor> getProfessors() {
        return employees;
    }

    public String toString() {
        return "Department no: " + getId() + 
               ", name: " + getName();
    }
}

Download Query_AVG_Simple.zip

The code above generates the following result.

The following is the database dump.

Table Name: ADDRESS




Table Name: DEPARTMENT
 Row:
    Column Name: ID,
    Column Type: INTEGER:
    Column Value: 1

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Design





Table Name: PHONE
 Row:
    Column Name: ID,
    Column Type: BIGINT:
    Column Value: 1

    Column Name: NUMBER,
    Column Type: VARCHAR:
    Column Value: 111-111-1111

    Column Name: TYPE,
    Column Type: VARCHAR:
    Column Value: null

    Column Name: EMPLOYEE_ID,
    Column Type: INTEGER:
    Column Value: 0





Table Name: PROFESSOR
 Row:
    Column Name: ID,
    Column Type: INTEGER:
    Column Value: 0

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: TOM

    Column Name: SALARY,
    Column Type: BIGINT:
    Column Value: 1111

    Column Name: STARTDATE,
    Column Type: DATE:
    Column Value: null

    Column Name: ADDRESS_ID,
    Column Type: INTEGER:
    Column Value: null

    Column Name: DEPARTMENT_ID,
    Column Type: INTEGER:
    Column Value: 1

    Column Name: MANAGER_ID,
    Column Type: INTEGER:
    Column Value: null





Table Name: PROJECT




Table Name: PROJECT_PROFESSOR

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