JPA条件 GROUP BY子句 JPA条件GROUPBY子句|JPA

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 【JPA条件 GROUP BY子句】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：-

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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 {@Idprivate 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="http://www.srcmini.com/com.mysql.jdbc.Driver"/> < property name="javax.persistence.jdbc.url" value="http://www.srcmini.com/jdbc:mysql://localhost:3306/studentdata"/> < property name="javax.persistence.jdbc.user" value="http://www.srcmini.com/root"/> < property name="javax.persistence.jdbc.password" value=""/> < property name="eclipselink.logging.level" value="http://www.srcmini.com/SEVERE"/> < property name="eclipselink.ddl-generation" value="http://www.srcmini.com/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：