Null Expression Example
In this section, you will see the example of null expression in with JPQL (Java Persistence Query Language). In this example, we will use a JPQL query ("SELECT st FROM Student st WHERE st.scourse IS NULL"). This query retrieve all record that contains "scourse" file is null.
You need the following artifacts:
- Database Table: student
- Model Class: Student.java
- Main Class: JPANullExpression.java
Database Table: student
CREATE TABLE `student` ( `id` int(11) NOT NULL auto_increment, `sname` varchar(40) NOT NULL, `sroll` int(11) NOT NULL, `scourse` varchar(10) default NULL, PRIMARY KEY (`id`) ) |
Model Class: Student.java
/* * To change this template, choose Tools | Templates * and open the template in the editor. */ package jpacrud; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.Table; /** * * @author Administrator */ @Entity @Table(name="student") public class Student implements java.io.Serializable { @Id @GeneratedValue private int id; public int getId() { return id; } public void setId(int id) { this.id = id; } @Column(name="sname",length=40,nullable=false) private String sname; public String getSname() { return sname; } public void setSname(String sname) { this.sname = sname; } @Column(name="sroll",nullable=false) private int sroll; public int getSroll() { return sroll; } public void setSroll(int sroll) { this.sroll = sroll; } @Column(name="scourse",length=10,nullable=true) private String scourse; public String getScourse() { return scourse; } public void setScourse(String scourse) { this.scourse = scourse; } } |
Main Class: JPANullExpression.java
/* * To change this template, choose Tools | Templates * and open the template in the editor. */ package jpacrud; import java.util.Iterator; import java.util.List; import javax.persistence.EntityManager; import javax.persistence.EntityManagerFactory; import javax.persistence.EntityTransaction; import javax.persistence.Persistence; import javax.persistence.Query; /** * * @author Administrator */ public class JPANullExpression { public static void main(String arg[]){ EntityManagerFactory emf=Persistence.createEntityManagerFactory("netjpa"); EntityManager em=emf.createEntityManager(); try{ EntityTransaction entr=em.getTransaction(); entr.begin(); Query query=em.createQuery("SELECT st FROM Student st WHERE st.scourse IS NULL"); List stEmtyList=query.getResultList(); Iterator stEmptyIterator=stEmtyList.iterator(); while(stEmptyIterator.hasNext()){ Student stu=(Student)stEmptyIterator.next(); System.out.print("Name:"+stu.getSname()); System.out.print(" Roll:"+stu.getSroll()); System.out.print(" Course:"+stu.getScourse()); System.out.println(); } entr.commit(); } catch(Exception ex){ System.out.println(ex.getMessage()); } finally{ em.close(); } } } |
Output:
init: deps-jar: compile-single: run-single: log4j:WARN No appenders could be found for logger (org.hibernate.cfg.annotations.Version). log4j:WARN Please initialize the log4j system properly. Hibernate: select student0_.id as id0_, student0_.scourse as scourse0_, student0_.sname as sname0_, student0_.sroll as sroll0_ from student student0_ where student0_.scourse is null Name:Suman Roll:104 Course:null Name:Noor Roll:105 Course:null BUILD SUCCESSFUL (total time: 3 seconds) |