Hibernate（35）什么是Hibernate的聚合函数？

Hibernate的聚合函数

在Hibernate中，聚合函数（Aggregate Functions）用于对一组值进行计算并返回单一值。这些函数在执行统计分析、数据汇总和报告时非常有用。常见的聚合函数包括：

count：计数
avg：平均值
sum：求和
min：最小值
max：最大值

Hibernate支持在Criteria API和HQL（Hibernate Query Language）中使用聚合函数。

使用聚合函数的示例代码

实体类定义

我们继续使用之前定义的Person和Address实体类。

Person类

java 复制代码

package com.example.domain;

import javax.persistence.*;

@Entity
@Table(name = "person")
public class Person {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name")
    private String name;

    @Column(name = "age")
    private int age;

    public Person() {}

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // Getters 和 Setters

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

Address类

java 复制代码

package com.example.domain;

import javax.persistence.*;

@Entity
@Table(name = "address")
public class Address {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "street")
    private String street;

    @ManyToOne
    @JoinColumn(name = "person_id", nullable = false)
    private Person person;

    public Address() {}

    public Address(String street, Person person) {
        this.street = street;
        this.person = person;
    }

    // Getters 和 Setters

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getStreet() {
        return street;
    }

    public void setStreet(String street) {
        this.street = street;
    }

    public Person getPerson() {
        return person;
    }

    public void setPerson(Person person) {
        this.person = person;
    }
}

Hibernate配置文件 `hibernate.cfg.xml`

xml 复制代码

<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <!-- 数据库连接配置 -->
        <property name="hibernate.connection.driver_class">com.mysql.cj.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/your_database</property>
        <property name="hibernate.connection.username">your_username</property>
        <property name="hibernate.connection.password">your_password</property>

        <!-- Hibernate 属性配置 -->
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="hibernate.show_sql">true</property>
        <property name="hibernate.format_sql">true</property>
        <property name="hibernate.hbm2ddl.auto">update</property>

        <!-- 映射类 -->
        <mapping class="com.example.domain.Person"/>
        <mapping class="com.example.domain.Address"/>
    </session-factory>
</hibernate-configuration>

HibernateUtil类

java 复制代码

import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class HibernateUtil {
    private static final SessionFactory sessionFactory;

    static {
        try {
            // 从配置文件创建SessionFactory
            sessionFactory = new Configuration().configure("hibernate.cfg.xml").buildSessionFactory();
        } catch (Throwable ex) {
            // 记录启动失败的错误
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }
}

使用聚合函数的示例

插入示例数据

java 复制代码

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;

public class HibernateInsertData {
    public static void main(String[] args) {
        // 获取SessionFactory
        SessionFactory sessionFactory = HibernateUtil.getSessionFactory();

        // 插入示例数据
        insertSampleData(sessionFactory);

        // 关闭SessionFactory
        sessionFactory.close();
    }

    private static void insertSampleData(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        Transaction transaction = session.beginTransaction();
        try {
            Person person1 = new Person("John Doe", 30);
            Person person2 = new Person("Jane Doe", 28);
            Person person3 = new Person("Alice", 35);

            session.save(person1);
            session.save(person2);
            session.save(person3);

            Address address1 = new Address("123 Main St", person1);
            Address address2 = new Address("456 Oak St", person2);
            Address address3 = new Address("789 Pine St", person3);

            session.save(address1);
            session.save(address2);
            session.save(address3);

            transaction.commit();
            System.out.println("Inserted sample data");
        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }
}

使用聚合函数查询

查询Person的平均年龄

java 复制代码

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.query.criteria.HibernateCriteriaBuilder;
import org.hibernate.query.criteria.JpaCriteriaQuery;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;

public class HibernateAggregateQueryExample {
    public static void main(String[] args) {
        // 获取SessionFactory
        SessionFactory sessionFactory = HibernateUtil.getSessionFactory();

        // 使用聚合函数查询Person的平均年龄
        queryAverageAge(sessionFactory);

        // 关闭SessionFactory
        sessionFactory.close();
    }

    private static void queryAverageAge(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        try {
            CriteriaBuilder criteriaBuilder = session.getCriteriaBuilder();
            CriteriaQuery<Double> criteriaQuery = criteriaBuilder.createQuery(Double.class);
            Root<Person> root = criteriaQuery.from(Person.class);

            criteriaQuery.select(criteriaBuilder.avg(root.get("age")));

            Double averageAge = session.createQuery(criteriaQuery).getSingleResult();
            System.out.println("Average age of persons: " + averageAge);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }
}

查询Person的总数

java 复制代码

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.query.criteria.HibernateCriteriaBuilder;
import org.hibernate.query.criteria.JpaCriteriaQuery;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;

public class HibernateCountQueryExample {
    public static void main(String[] args) {
        // 获取SessionFactory
        SessionFactory sessionFactory = HibernateUtil.getSessionFactory();

        // 使用聚合函数查询Person的总数
        queryPersonCount(sessionFactory);

        // 关闭SessionFactory
        sessionFactory.close();
    }

    private static void queryPersonCount(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        try {
            CriteriaBuilder criteriaBuilder = session.getCriteriaBuilder();
            CriteriaQuery<Long> criteriaQuery = criteriaBuilder.createQuery(Long.class);
            Root<Person> root = criteriaQuery.from(Person.class);

            criteriaQuery.select(criteriaBuilder.count(root));

            Long personCount = session.createQuery(criteriaQuery).getSingleResult();
            System.out.println("Total number of persons: " + personCount);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }
}

查询年龄最小和最大的Person

java 复制代码

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.query.criteria.HibernateCriteriaBuilder;
import org.hibernate.query.criteria.JpaCriteriaQuery;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;

public class HibernateMinMaxQueryExample {
    public static void main(String[] args) {
        // 获取SessionFactory
        SessionFactory sessionFactory = HibernateUtil.getSessionFactory();

        // 使用聚合函数查询年龄最小和最大的Person
        queryMinMaxAge(sessionFactory);

        // 关闭SessionFactory
        sessionFactory.close();
    }

    private static void queryMinMaxAge(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        try {
            CriteriaBuilder criteriaBuilder = session.getCriteriaBuilder();

            // 查询最小年龄
            CriteriaQuery<Integer> minCriteriaQuery = criteriaBuilder.createQuery(Integer.class);

Hibernate（35）什么是Hibernate的聚合函数？

Hibernate的聚合函数

使用聚合函数的示例代码

实体类定义

Person类

Address类

Hibernate配置文件 hibernate.cfg.xml

HibernateUtil类

使用聚合函数的示例

插入示例数据

使用聚合函数查询

查询Person的平均年龄

查询Person的总数

查询年龄最小和最大的Person

Hibernate配置文件 `hibernate.cfg.xml`