在Java的Hibernate框架中对数据库数据进行查询操作

网友投稿 175 2023-07-24


在Java的Hibernate框架中对数据库数据进行查询操作

Hibernate查询语言(HQL)是一种面向对象的查询语言,类似于SQL,但不是对表和列操作,HQL适用于持久对象和它们的属性。 HQL查询由Hibernate转换成传统的SQL查询,这在圈上的数据库执行操作。

虽然可以直接使用SQL语句和Hibernate使用原生SQL,但建议使用HQL尽可能避免数据库可移植性的麻烦,并采取Hibernate的SQL生成和缓存策略的优势。

都像SELECT,FROM和WHERE等关键字不区分大小写,但如表名和列名的属性是区分在HQL敏感。

FROM 语句

使用FROM子句,如果要加载一个完整的持久化对象到内存中。下面是一个使用FROM子句的简单的语法:

String hql = "FROM Employee";

Query query = session.createQuery(hql);

List results = query.list();

如果需要完全限定在HQL一个类名,只需指定如下的包和类名:

String hql = "FROM com.hibernatebook.criteria.Employee";

Query query = session.createQuery(hql);

List results = query.list();

AS 语句

AS子句可以用来别名分配给类中的HQL查询,特别是当有很长的查询。例如,我们前面简单的例子是以下几点:

String hql = "FROM Employee AS E";

Query query = session.createQuery(hql);

List results = query.list();

AS关键字是可选的,也可以直接在之后的类名指定别名,如下所示:

String hql = "FROM Employee E";

Query query = session.createQuery(hql);

List results = query.list();

SELECT 子句

SELECT子句提供了更多的控制权比from子句的结果集。如果想获得对象而不是整个对象的几个属性,使用SELECT子句。下面是一个使用SELECT语句来获取Employee对象只是FIRST_NAME字段的简单的语法:

String hql = "SELECT E.firstName FROM Employee E";

Query query = session.createQuery(hql);

List results = query.list();

值得注意的是在这里,Employee.firstName是Employee对象的一个属性,而不是EMPLOYEE表的一个字段。

WHERE 子句

如果想缩小了从存储返回的特定对象,可以使用WHERE子句。下面是一个使用WHERE子句的简单的语法:

String hql = "FROM Employee E WHERE E.id = 10";

Query query = session.createQuery(hql);

List results = query.list();

ORDER BY 子句

若要排序HQL查询的结果,将需要使用ORDER BY子句。您可以在结果集按升序(ASC)或降序(DESC)通过在对象的任何属性排序结果。下面是一个使用ORDER BY子句的简单的语法:

String hql = "FROM Employee E WHERE E.id > 10 ORDER BY E.salary DESC";

Query query = session.createQuery(hql);

List results = query.list();

如果想通过一个以上的属性进行排序,你会仅仅是额外的属性添加到由子句用逗号隔开,如下所示的命令的结尾:

String hql = "FROM Employee E WHERE E.id > 10 " +

"ORDER BY E.firstName DESC, E.salary DESC ";

Query query = session.createQuery(hql);

List results = query.list();

GROUP BY 子句

该子句允许从Hibernate的它基于属性的值的数据库和组提取信息,并且通常使用结果包括总值。下面是一个使用GROUP BY子句的语法很简单:

String hql = "SELECT SUM(E.salary), E.firtName FROM Employee E " +

"GROUP BY E.firstName";

Query query = session.createQuery(hql);

List results = query.list();

使用命名参数

Hibernate命名在其HQL查询参数支持。这使得编写接受来自用户的输入容易,不必对SQL注入攻击防御HQL查询。下面是一个使用命名参数的简单的语法:

String hql = "FROM Employee E WHERE E.id = :employee_id";

Query query = session.createQuery(hql);

query.setParameter("employee_id",10);

List results = query.list();

UPDATE 子句

批量更新是新的HQL与Hibernate3,以及不同的删除工作,在Hibernate 3和Hibernate2一样。 Query接口现在包含一个名为executeUpdate()方法用于执行HQL UPDATE或DELETE语句。

在UPDATE子句可以用于更新一个或多个对象中的一个或多个属性。下面是一个使用UPDATE子句的简单的语法:

String hql = "UPDATE Employee set salary = :salary " +

"WHERE id = :employee_id";

Query query = session.createQuery(hql);

query.setParameter("salary", 1000);

query.setParameter("employee_id", 10);

int result = query.executeUpdate();

System.out.println("Rows affected: " + result);

DELETE 子句

DELETE子句可以用来删除一个或多个对象。下面是一个使用DELETE子句的简单的语法:

String hql = "DELETE FROM Employee " +

"WHERE id = :employee_id";

Query query = session.createQuery(hql);

query.setParameter("employee_id", 10);

int result = query.executeUpdate();

System.out.println("Rows affected: " + result);

INSERT 子句

HQL支持INSERT INTO子句中只记录在那里可以插入从一个对象到另一个对象。以下是使用INSERT INTO子句的简单的语法:

String hql = "INSERT INTO Employee(firstName, lastName, salary)" +

"SELECT firstName, lastName, salary FROM old_employee";

Query query = session.createQuery(hql);

int result = query.executeUpdate();

System.out.println("Rows affected: " + result);

聚合方法

HQL支持多种聚合方法,类似于SQL。他们工作在HQL同样的方式在SQL和下面的可用功能列表:

DISTINCT关键字只计算在该行设定的唯一值。下面的查询将只返回唯一的计数:

String hql = "SELECT count(distinct E.firstName) FROM Employee E";

Query query = session.createQuery(hql);

List results = query.list();

使用查询分页

有用于分页查询接口的两个方法。

Query setFirstResult(int startPosition)

Query setMaxResults(int maxResult)

采用上述两种方法一起,可以在网站或Swing应用程序构建一个分页组件。下面是例子,可以扩展来获取10行:

String hql = "FROM Employee";

Query query = session.createQuery(hql);

query.setFirstResult(1);

query.setMaxResults(10);

List results = query.list();

查询条件

Hibernate提供了操作对象,并依次数据在RDBMS表可用的备用方式。其中一个方法是标准的API,它允许你建立一个标准的查询对象编程,可以套用过滤规则和逻辑条件。

Hibernate的Session接口提供了可用于创建一个返回的持久化对象的类的实例时,应用程序执行一个条件查询一个Criteria对象createCriteria()方法。

以下是最简单的一个条件查询的例子是将简单地返回对应于Employee类的每个对象。

Criteria cr = session.createCriteria(Employee.class);

List results = cr.list();

限制与标准:

可以使用add()方法可用于Criteria对象添加限制条件查询。下面是例子增加一个限制与薪水返回的记录是等于2000:

Criteria cr = session.createCriteria(Employee.class);

cr.add(Restrictions.eq("salary", 2000));

List results = cr.list();

以下是几个例子覆盖不同的场景,并且可以根据要求使用:

Criteria cr = session.createCriteria(Employee.class);

// To get records having salary more than 2000

cr.add(Restrictions.gt("salary", 2000));

// To get records having salary less than 2000

cr.add(Restrictions.lt("salary", 2000));

// To get records having fistName starting with zara

cr.add(Restrictions.like("firstName", "zara%"));

// Case sensitive form of the above restriction.

cr.add(Restrictions.ilike("firstName", "zara%"));

// To get records having salary in between 1000 and 2000

cr.add(Restrictions.between("salary", 1000, 2000));

// To check if the given property is null

cr.add(Restrictions.isNull("salary"));

// To check if the given property is not null

cr.add(Restrictions.isNotNull("salary"));

// To check if the given property is empty

cr.add(Restrictions.isEmpty("salary"));

// To check if the given property is not empty

cr.add(Restrictions.isNotEmpty("salary"));

可以创建AND或OR使用LogicalExpression限制如下条件:

Criteria cr = session.createCriteria(Employee.class);

Criterion salary = Restrictions.gt("salary", 2000);

Criterion name = Restrictions.ilike("firstNname","zara%");

// To get records matching with OR condistions

LogicalExpression orExp = Restrictions.or(salary, name);

cr.add( orExp );

// To get records matching with AND condistions

LogicalExpression andExp = Restrictions.and(salary, name);

cr.add( andExp );

List results = cr.list();

虽然上述所有条件,可以直接使用HQL在前面的教程中介绍。

分页使用标准:

还有的标准接口,用于分页的两种方法。

public Criteria setFirstResult(int firstResult)

public Criteria setMaxResults(int maxResults)

采用上述两种方法一起,我们可以在我们的网站或Swing应用程序构建一个分页组件。下面是例子,可以扩展来每次获取10行:

Criteria cr = session.createCriteria(Employee.class);

cr.setFirstResult(1);

cr.setMaxResults(10);

List results = cr.list();

排序的结果:

标准的API提供了org.hibernate.criterion.Order类排序按升序或降序排列你的结果集,根据对象的属性。这个例子演示了如何使用Order类的结果集进行排序:

Criteria cr = session.createCriteria(Employee.class);

// To get records having salary more than 2000

cr.add(Restrictions.gt("salary", 2000));

// To sort records in descening order

crit.addOrder(Order.desc("salary"));

// To sort records in ascending order

crit.addOrder(Order.asc("salary"));

List results = cr.list();

预测与聚合:

该Criteria API提供了一个org.hibernate.criterion.Projections类可用于获取平均值,最大值或最小值的属性值。Projections类是类似于类限制,因为它提供了几个静态工厂方法用于获得Projection 实例。  provides the

以下是涉及不同的方案的一些例子,可按规定使用:

Criteria cr = session.createCriteria(Employee.class);

// To get total row count.

cr.setProjection(Projections.rowCount());

// To get average of a property.

cr.setProjection(Projections.avg("salary"));

// To get distinct count of a property.

cr.setProjection(Projections.countDistinct("firstName"));

// To get maximum of a property.

cr.setProjection(Projections.max("salary"));

// To get minimum of a property.

cr.setProjection(Projections.min("salary"));

// To get sum of a property.

cr.setProjection(Projections.sum("salary"));

Criteria Queries 例子:

考虑下面的POJO类:

public class Employee {

private int id;

private String firstName;

private String lastName;

private int salary;

public Employee() {}

public Employee(String fname, String lname, int salary) {

this.firstName = fname;

this.lastName = lname;

this.salary = salary;

}

public int getId() {

return id;

}

public void setId( int id ) {

this.id = id;

}

public String getFirstName() {

return firstName;

}

public void setFirstName( String first_name ) {

this.firstName = first_name;

}

public String getLastName() {

return lastName;

}

public void setLastName( String last_name ) {

this.lastName = last_name;

}

public int getSalary() {

return salary;

}

public void setSalary( int salary ) {

this.salary = salary;

}

}

让我们创建下面的EMPLOYEE表来存储Employee对象:

create table EMPLOYEE (

id INT NOT NULL auto_increment,

first_name VARCHAR(20) default NULL,

last_name VARCHAR(20) default NULL,

salary INT default NULL,

PRIMARY KEY (id)

);

以下将被映射文件。

"-//Hibernate/Hibernate Mapping DTD//EN"

"http://hibernate.org/dtd/hibernate-mapping-3.0.dtd">

This class contains the employee detail.

最后,我们将创建应用程序类的main()方法来运行,我们将使用Criteria查询的应用程序:

import java.util.List;

import java.util.Date;

import java.util.Iterator;

import org.hibernate.HibernateException;

import org.hibernate.Session;

import org.hibernate.Transaction;

import org.hibernate.SessionFactory;

import org.hibernate.Criteria;

import org.hibernate.criterion.Restrictions;

import org.hibernate.criterion.Projections;

import org.hibernate.cfg.Configuration;

public class ManageEmployee {

private static SessionFactory factory;

public static void main(String[] args) {

try{

factory = new Configuration().configure().buildSessionFactory();

}catch (Throwable ex) {

System.err.println("Failed to create sessionFactory object." + ex);

throw new ExceptionInInitializerError(ex);

}

ManageEmployee ME = new ManageEmployee();

/* Add few employee records in database */

Integer empID1 = ME.addEmployee("Zara", "Ali", 2000);

Integer empID2 = ME.addEmployee("Daisy", "Das", 5000);

Integer empID3 = ME.addEmployee("John", "Paul", 5000);

Integer empID4 = ME.addEmployee("Mohd", "Yasee", 3000);

/* List down all the employees */

ME.listEmployees();

/* Print Total employee's count */

ME.countEmployee();

/* Print Toatl salary */

ME.totalSalary();

}

/* Method to CREATE an employee in the database */

public Integer addEmployee(String fname, String lname, int salary){

Session session = factory.openSession();

Transaction tx = null;

Integer employeeID = null;

try{

tx = session.beginTransaction();

Employee employee = new Employee(fname, lname, salary);

employeeID = (Integer) session.save(employee);

tx.commit();

}catch (HibernateException e) {

if (tx!=null) tx.rollback();

e.printStackTrace();

}finally {

session.close();

}

return employeeID;

}

/* Method to READ all the employees having salary more than 2000 */

public void listEmployees( ){

Session session = factory.openSession();

Transaction tx = null;

try{

tx = session.beginTransaction();

Criteria cr = session.createCriteria(Employee.class);

// Add restriction.

cr.add(Restrictions.gt("salary", 2000));

List employees = cr.list();

for (Iterator iterator =

employees.iterator(); iterator.hasNext();){

Employee employee = (Employee) iterator.next();

System.out.print("First Name: " + employee.getFirstName());

System.out.print(" Last Name: " + employee.getLastName());

System.out.println(" Salary: " + employee.getSalary());

}

tx.commit();

}catch (HibernateException e) {

if (tx!=null) tx.rollback();

e.printStackTrace();

}finally {

session.close();

}

}

/* Method to print total number of records */

public void countEmployee(){

Session session = factory.openSession();

Transaction tx = null;

try{

tx = session.beginTransaction();

Criteria cr = session.createCriteria(Employee.class);

// To get total row count.

cr.setProjection(Projections.rowCount());

List rowCount = cr.list();

System.out.println("Total Coint: " + rowCount.get(0) );

tx.commit();

}catch (HibernateException e) {

if (tx!=null) tx.rollback();

e.printStackTrace();

}finally {

session.close();

}

}

/* Method to print sum of salaries */

public void totalSalary(){

Session session = factory.openSession();

Transaction tx = null;

try{

tx = session.beginTransaction();

Criteria cr = session.createCriteria(Employee.class);

// To get total salary.

cr.setProjection(Projections.sum("salary"));

List totalSalary = cr.list();

System.out.println("Total Salary: " + totalSalary.get(0) );

tx.commit();

}catch (HibernateException e) {

if (tx!=null) tx.rollback();

e.printStackTrace();

}finally {

session.close();

}

}

}

编译和执行:

下面是步骤来编译并运行上述应用程序。请确保您已在进行的编译和执行之前,适当地设置PATH和CLASSPATH。

创建hibernate.cfg.xml配置文件中配置章节解释。

创建Employee.hbm.xml映射文件,如上图所示。

创建Employee.java源文件,如上图所示,并编译它。

创建ManageEmployee.java源文件,如上图所示,并编译它。

执行ManageEmployee二进制运行程序.

会得到以下结果,并记录将创建在EMPLOYEE表中。

$java ManageEmployee

.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........

First Name: Daisy Last Name: Das Salary: 5000

First Name: John Last Name: Paul Salary: 5000

First Name: Mohd Last Name: Yasee Salary: 3000

Total Coint: 4

Total Salary: 15000

如果检查EMPLOYEE表,它应该记录如下:

mysql> select * from EMPLOYEE;

+----+------------+-----------+--------+

| id | first_name | last_name | salary |

+---mbptegUT-+------------+-----------+--------+

| 14 | Zara | Ali | 2000 |

| 15 | Daisy | Das | 5000 |

| 16 | John | Paul | 5000 |

| 17 | Mohd | Yasee | 3000 |

+----+------------+-----------+--------+

4 rows in set (0.00 sec)


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