Hibernate查詢語言（HQL）是一種面向?qū)ο蟮牟樵冋Z言，類似于SQL，但不是對表和列操作，HQL適用于持久對象和它們的屬性。 HQL查詢由Hibernate轉(zhuǎn)換成傳統(tǒng)的SQL查詢，這在圈上的數(shù)據(jù)庫執(zhí)行操作。
雖然可以直接使用SQL語句和Hibernate使用原生SQL，但建議使用HQL盡可能避免數(shù)據(jù)庫可移植性的麻煩，并采取Hibernate的SQL生成和緩存策略的優(yōu)勢。
都像SELECT，F(xiàn)ROM和WHERE等關(guān)鍵字不區(qū)分大小寫，但如表名和列名的屬性是區(qū)分在HQL敏感。
FROM 語句
使用FROM子句，如果要加載一個完整的持久化對象到內(nèi)存中。下面是一個使用FROM子句的簡單的語法：
String hql = "FROM Employee";Query query = session.createQuery(hql);List results = query.list();
String hql = "FROM com.hibernatebook.criteria.Employee";Query query = session.createQuery(hql);List results = query.list();
AS 語句
AS子句可以用來別名分配給類中的HQL查詢，特別是當有很長的查詢。例如，我們前面簡單的例子是以下幾點：
String hql = "FROM Employee AS E";Query query = session.createQuery(hql);List results = query.list();
AS關(guān)鍵字是可選的，也可以直接在之后的類名指定別名，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權(quán)比from子句的結(jié)果集。如果想獲得對象而不是整個對象的幾個屬性，使用SELECT子句。下面是一個使用SELECT語句來獲取Employee對象只是FIRST_NAME字段的簡單的語法：
String hql = "SELECT E.firstName FROM Employee E";Query query = session.createQuery(hql);List results = query.list();
值得注意的是在這里，Employee.firstName是Employee對象的一個屬性，而不是EMPLOYEE表的一個字段。
WHERE 子句
如果想縮小了從存儲返回的特定對象，可以使用WHERE子句。下面是一個使用WHERE子句的簡單的語法：
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查詢的結(jié)果，將需要使用ORDER BY子句。您可以在結(jié)果集按升序（ASC）或降序（DESC）通過在對象的任何屬性排序結(jié)果。下面是一個使用ORDER BY子句的簡單的語法：
String hql = "FROM Employee E WHERE E.id > 10 ORDER BY E.salary DESC";Query query = session.createQuery(hql);List results = query.list();
如果想通過一個以上的屬性進行排序，你會僅僅是額外的屬性添加到由子句用逗號隔開，如下所示的命令的結(jié)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
該子句允許從Hibernate的它基于屬性的值的數(shù)據(jù)庫和組提取信息，并且通常使用結(jié)果包括總值。下面是一個使用GROUP BY子句的語法很簡單：
String hql = "SELECT SUM(E.salary), E.firtName FROM Employee E " + "GROUP BY E.firstName";Query query = session.createQuery(hql);List results = query.list();
使用命名參數(shù)
Hibernate命名在其HQL查詢參數(shù)支持。這使得編寫接受來自用戶的輸入容易，不必對SQL注入攻擊防御HQL查詢。下面是一個使用命名參數(shù)的簡單的語法：
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與Hibernate3，以及不同的刪除工作，在Hibernate 3和Hibernate2一樣。 Query接口現(xiàn)在包含一個名為executeUpdate()方法用于執(zhí)行HQL UPDATE或DELETE語句。
在UPDATE子句可以用于更新一個或多個對象中的一個或多個屬性。下面是一個使用UPDATE子句的簡單的語法：
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子句可以用來刪除一個或多個對象。下面是一個使用DELETE子句的簡單的語法：
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子句中只記錄在那里可以插入從一個對象到另一個對象。以下是使用INSERT INTO子句的簡單的語法：
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支持多種聚合方法，類似于SQL。他們工作在HQL同樣的方式在SQL和下面的可用功能列表：
DISTINCT關(guān)鍵字只計算在該行設(shè)定的唯一值。下面的查詢將只返回唯一的計數(shù)：
String hql = "SELECT count(distinct E.firstName) FROM Employee E";Query query = session.createQuery(hql);List results = query.list();
使用查詢分頁
有用于分頁查詢接口的兩個方法。
采用上述兩種方法一起，可以在網(wǎng)站或Swing應(yīng)用程序構(gòu)建一個分頁組件。下面是例子，可以擴展來獲取10行：
String hql = "FROM Employee";Query query = session.createQuery(hql);query.setFirstResult(1);query.setMaxResults(10);List results = query.list();
查詢條件
Hibernate提供了操作對象，并依次數(shù)據(jù)在RDBMS表可用的備用方式。其中一個方法是標準的API，它允許你建立一個標準的查詢對象編程，可以套用過濾規(guī)則和邏輯條件。
Hibernate的Session接口提供了可用于創(chuàng)建一個返回的持久化對象的類的實例時，應(yīng)用程序執(zhí)行一個條件查詢一個Criteria對象createCriteria()方法。
以下是最簡單的一個條件查詢的例子是將簡單地返回對應(yīng)于Employee類的每個對象。
Criteria cr = session.createCriteria(Employee.class);List results = cr.list();
限制與標準：
可以使用add（）方法可用于Criteria對象添加限制條件查詢。下面是例子增加一個限制與薪水返回的記錄是等于2000：
Criteria cr = session.createCriteria(Employee.class);cr.add(Restrictions.eq("salary", 2000));List results = cr.list();以下是幾個例子覆蓋不同的場景，并且可以根據(jù)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"));可以創(chuàng)建AND或OR使用LogicalExpression限制如下條件：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();雖然上述所有條件，可以直接使用HQL在前面的教程中介紹。
分頁使用標準：
還有的標準接口，用于分頁的兩種方法。
采用上述兩種方法一起，我們可以在我們的網(wǎng)站或Swing應(yīng)用程序構(gòu)建一個分頁組件。下面是例子，可以擴展來每次獲取10行：
Criteria cr = session.createCriteria(Employee.class);cr.setFirstResult(1);cr.setMaxResults(10);List results = cr.list();
排序的結(jié)果：
標準的API提供了org.hibernate.criterion.Order類排序按升序或降序排列你的結(jié)果集，根據(jù)對象的屬性。這個例子演示了如何使用Order類的結(jié)果集進行排序：
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();預測與聚合：
該Criteria API提供了一個org.hibernate.criterion.Projections類可用于獲取平均值，最大值或最小值的屬性值。Projections類是類似于類限制，因為它提供了幾個靜態(tài)工廠方法用于獲得Projection 實例。 provides the
以下是涉及不同的方案的一些例子，可按規(guī)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：
考慮下面的POJO類：
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; }}讓我們創(chuàng)建下面的EMPLOYEE表來存儲Employee對象：
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));
以下將被映射文件。
<?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，我們將創(chuàng)建應(yīng)用程序類的main（）方法來運行，我們將使用Criteria查詢的應(yīng)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(); } }}編譯和執(zhí)行：
下面是步驟來編譯并運行上述應(yīng)用程序。請確保您已在進行的編譯和執(zhí)行之前，適當?shù)卦O(shè)置PATH和CLASSPATH。
會得到以下結(jié)果，并記錄將創(chuàng)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
如果檢查EMPLOYEE表，它應(yīng)該記錄如下：
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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