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在Java的Hibernate框架中对数据库数据进行查询操作

2019-11-26 14:44:11
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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:

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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 2000cr.add(Restrictions.gt("salary", 2000));// To get records having salary less than 2000cr.add(Restrictions.lt("salary", 2000));// To get records having fistName starting with zaracr.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 2000cr.add(Restrictions.between("salary", 1000, 2000));// To check if the given property is nullcr.add(Restrictions.isNull("salary"));// To check if the given property is not nullcr.add(Restrictions.isNotNull("salary"));// To check if the given property is emptycr.add(Restrictions.isEmpty("salary"));// To check if the given property is not emptycr.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 condistionsLogicalExpression orExp = Restrictions.or(salary, name);cr.add( orExp );// To get records matching with AND condistionsLogicalExpression 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 2000cr.add(Restrictions.gt("salary", 2000));// To sort records in descening ordercrit.addOrder(Order.desc("salary"));// To sort records in ascending ordercrit.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"?><!DOCTYPE hibernate-mapping PUBLIC  "-//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: 5000First Name: John Last Name: Paul Salary: 5000First Name: Mohd Last Name: Yasee Salary: 3000Total Coint: 4Total 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)
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