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。

xml version = "1.0" encoding = "utf-8" ?>

 "-//Hibernate/Hibernate Mapping DTD//EN"
 "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

< hibernate-mapping >
  < class name = "Employee" table = "EMPLOYEE" >
   < meta attribute = "class-description" >
     This class contains the employee detail.
   meta >
   < id name = "id" type = "int" column = "id" >
     < generator class = "native" />
   id >
   < property name = "firstName" column = "first_name" type = "string" />
   < property name = "lastName" column = "last_name" type = "string" />
   < property name = "salary" column = "salary" type = "int" />
  class >
hibernate-mapping >

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 |
+----+------------+-----------+--------+
| 14 | Zara    | Ali    |  2000 |
| 15 | Daisy   | Das    |  5000 |
| 16 | John    | Paul   |  5000 |
| 17 | Mohd    | Yasee   |  3000 |
+----+------------+-----------+--------+
4 rows in set (0.00 sec)