JPA标准的Having子句

HAVING子句与GROUP BY子句一起使用以过滤表中的数据。在Criteria API中, 使用Abstract接口的hading()方法来设置分组数据的条件。

条件示例

在这里, 我们将对学生表执行具有HAVING操作的GROUP BY。让我们假设该表包含以下记录：-

现在, 请按照以下步骤执行操作：-

• 在com.srcmini.jpa包下创建一个实体类名称为StudentEntity.java。此类包含带有所有必需注释的三个属性s_id, s_name和s_age。

StudentEntity.java

package com.srcmini.jpa;
	import javax.persistence.*;

	@Entity
	@Table(name="student")
	public class StudentEntity {

		@Id
		private int s_id;
		private String s_name;
		private int s_age;
		
		public StudentEntity(int s_id, String s_name, int s_age) {
			super();
			this.s_id = s_id;
			this.s_name = s_name;
			this.s_age = s_age;
		}

		public StudentEntity() {
			super();
		}

		public int getS_id() {
			return s_id;
		}

		public void setS_id(int s_id) {
			this.s_id = s_id;
		}

		public String getS_name() {
			return s_name;
		}

		public void setS_name(String s_name) {
			this.s_name = s_name;
		}

		public int getS_age() {
			return s_age;
		}

		public void setS_age(int s_age) {
			this.s_age = s_age;
		}
		
	}

• 现在, 在Persistence.xml文件中映射实体类和其他数据库配置。

Persistence.xml

<persistence>
<persistence-unit name="Student_details">
   
      <class>com.srcmini.jpa.StudentEntity</class>

      <properties>
         <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
         <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/studentdata"/>
         <property name="javax.persistence.jdbc.user" value="root"/>
         <property name="javax.persistence.jdbc.password" value=""/>
         <property name="eclipselink.logging.level" value="SEVERE"/>
         <property name="eclipselink.ddl-generation" value="create-or-extend-tables"/>
      </properties>
      
   </persistence-unit>

</persistence>

• 一次, 我们创建了基本实体类并将配置映射到persistence.xml文件中, 我们可以执行不同类型的HAVING操作。在这里, 我们将年龄等于或大于24岁的学生人数分组。

StudentHaving.java

package com.srcmini.jpa.jpql;
import com.srcmini.jpa.StudentEntity;
import javax.persistence.*;
import javax.persistence.criteria.*;

import java.util.*;
public class StudentHaving {
	
	
	public static void main( String args[]) {
		   
		 
		 EntityManagerFactory emf = Persistence.createEntityManagerFactory( "Student_details" );
	      EntityManager em = emf.createEntityManager();
	      em.getTransaction().begin( );
	      

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Object[]> cq = cb.createQuery(Object[].class);
Root<StudentEntity> stud = cq.from(StudentEntity.class);

cq.multiselect(stud.get("s_age"), cb.count(stud)).groupBy(stud.get("s_age")).having(cb.ge(stud.get("s_age"), 24));
     
System.out.print("s_age");
System.out.println("\t Count");
List<Object[]> list = em.createQuery(cq).getResultList();
for(Object[] object : list){
	System.out.println(object[0] + " " + object[1]);

}

em.getTransaction().commit();
	      em.close();
	      emf.close();  
	 }
}

输出：