GROUP BY子句用于从一个或多个表中收集数据并将它们排列在一组中。在Criteria API中, AbstractQuery接口的groupBy()方法用于过滤记录并将其分组。
条件GROUP BY示例
在这里, 我们将对学生表执行GROUP BY操作。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, 我们可以执行不同类型的GROUP BY操作。在这里, 我们根据年龄对学生人数进行分组。
StudentGroup.java
package com.srcmini.jpa.jpql;
import com.srcmini.jpa.StudentEntity;
import javax.persistence.*;
import javax.persistence.criteria.*;
import java.util.*;
public class StudentGroup {
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"));
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:
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