HQL：Hibernate Query Language

  • 提供更加丰富灵活、更为强大的查询能力
  • HQL更接近SQL语句查询语法
  • 面向对象的查询
    • "from Children where cid<?"　　：这里Children指的是类不是children表；cid指的是Children类中的对象而不是children表中的字段

HQL查询（单表）

以一对多关系映射为例（parent[1]<——>children[n]）

简单查询

实体类

1.Parent.java

 package com.qf.entity;

 import java.util.HashSet;

 import java.util.Set;

 public class Parent {

     private Long pid;

     private String pname;

     private Integer age;

     private Set<Children> childs = new HashSet<>();

     @Override

     public String toString() {

         return "Parent [pid=" + pid + ", pname=" + pname + ", age=" + age + "]";

     }

     public Long getPid() {

         return pid;

     }

     public void setPid(Long pid) {

         this.pid = pid;

     }

     public String getPname() {

         return pname;

     }

     public void setPname(String pname) {

         this.pname = pname;

     }

     public Integer getAge() {

         return age;

     }

     public void setAge(Integer age) {

         this.age = age;

     }

     public Set<Children> getChilds() {

         return childs;

     }

     public void setChilds(Set<Children> childs) {

         this.childs = childs;

     }

 }

2.Children.java

 package com.qf.entity;

 public class Children {

     private Long cid;

     private String cname;

     private Character sex;

     private Parent p;

     @Override

     public String toString() {

         return "Children [cid=" + cid + ", cname=" + cname + ", sex=" + sex + ", p=" + p + "]";

     }

     public Long getCid() {

         return cid;

     }

     public void setCid(Long cid) {

         this.cid = cid;

     }

     public String getCname() {

         return cname;

     }

     public void setCname(String cname) {

         this.cname = cname;

     }

     public Character getSex() {

         return sex;

     }

     public void setSex(Character sex) {

         this.sex = sex;

     }

     public Parent getP() {

         return p;

     }

     public void setP(Parent p) {

         this.p = p;

     }

 }

注：两个实体类的toString()方法重写时

  • Parent类的toString()方法如果包含Set<Children>的属性，那么Children类的toString()方法就不能包含Parent的属性，否则查询输出对象信息时会陷入死循环，导致StackOverflowError
package com.qf.util;

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.cfg.Configuration;

public class HibernateUtils {

	private static Configuration cfg ;

	private static SessionFactory factory;

	static {

		cfg = new Configuration().configure();

		factory = cfg.buildSessionFactory();

	}

	public static Session getCurrentSession() {

		return factory.getCurrentSession();

	}

}
@Test

/**

 * HQL简单查询

 */

public void queryByHQL() {

	Session session = HibernateUtils.getCurrentSession();

	Transaction tx = session.beginTransaction();

	Query query = session.createQuery("from Children");

	List<Children> list = query.list();

	for (Children children : list) {

		System.out.println(children);

	}

	tx.commit();

}

别名查询

String hql = "select c from Children c order by cid desc";

HQL中支持数据库的带有别名的查询

@Test

/**

 * HQL别名查询

 */

public void queryByHQL02() {

	Session session = HibernateUtils.getCurrentSession();

	Transaction tx = session.beginTransaction();

//		String hql = "select c from Children c";

	String hql = "select c from Children c order by cid desc";

	Query query = session.createQuery(hql);

	List<Children> list = query.list();

	for (Children children : list) {

		System.out.println(children);

	}

	tx.commit();

}

条件查询

参数绑定

  • 位置绑定

    String hql = "select c from Children c where cid<? and cname like ?";
    
Query query = session.createQuery(hql);
    
query.setParameter(0, 20L);
    
query.setParameter(1, "%张%");
    • hql中参数以"？"代替
    • 索引从0开始
  • 名称绑定 
    String hql = "select c from Children c where cid<:aaa and cname like :bbb";
    
Query query = session.createQuery(hql);
    
query.setParameter("aaa", 20L);
    
query.setParameter("bbb", "%张%");
    • hql中参数以 ":自定义名称"的形式代替
    • 不使用索引，使用（名称，值）的形式
@Test

/**

 * HQL条件查询

 */

public void queryByHQL03() {

	Session session = HibernateUtils.getCurrentSession();

	Transaction tx = session.beginTransaction();

	/*//一个条件（位置绑定）

	String hql = "select c from Children c where cid<?";

	Query query = session.createQuery(hql);

	query.setInteger(0, 3);*/

	//多个条件（位置绑定）

	/*String hql = "select c from Children c where cid<? and cname like ?";

	Query query = session.createQuery(hql);

	query.setParameter(0, 20L);

	query.setParameter(1, "%张%");*/

	//多个条件（名称绑定）

	String hql = "select c from Children c where cid<:aaa and cname like :bbb";

	Query query = session.createQuery(hql);

	query.setParameter("aaa", 20L);

	query.setParameter("bbb", "%张%");

	List<Children> list = query.list();

	for (Children children : list) {

		System.out.println(children);

	}

	tx.commit();

}

投影查询

查询表中部分字段

方式一：返回Object[ ]

@Test

/**

 * HQL简单查询

 */

public void queryByHQL() {

	Session session = HibernateUtils.getCurrentSession();

	Transaction tx = session.beginTransaction();

	Query query = session.createQuery("select cname,sex from Children");

	List<Object[]> list = query.list();

	for (Object[] objs : list) {

		System.out.println(Arrays.toString(objs));

	}

	tx.commit();

}

方式二：返回实体类对象

实体类

  • 添加无参构造方法
  • 添加有参构造方法（构造方法参数是你所要查询的属性）
package com.qf.entity;

public class Children {

	private Long cid;

	private String cname;

	private Character sex;

	private Parent p;

	public Children(String cname, Character sex) {

		super();

		this.cname = cname;

		this.sex = sex;

	}

	public Children() {

		super();

	}

        ......

}

测试方法

@Test

/**

 * HQL简单查询

 */

public void queryByHQL() {

	Session session = HibernateUtils.getCurrentSession();

	Transaction tx = session.beginTransaction();

	Query query = session.createQuery("select new Children(cname,sex) from Children");

	List<Children> list = query.list();

	for (Children children : list) {

		System.out.println(children);

	}

	tx.commit();

}

分页查询

  • query.setFirstResult(int start)：从第几条记录开始查询
  • query.setMaxResults(int num)：本页一共查询多少条记录
@Test

public void query() {

    Session session = HibernateUtils.getCurrentSession();

    Transaction tx = session.beginTransaction();

    String sql = "from Children";

    Query query = session.createQuery(sql);

    query.setFirstResult(2);

    query.setMaxResults(3);

    List<Children> list = query.list();

    for (Children c : list) {

        System.out.println(c);

    }

    tx.commit();

}

分组查询

max()、min()、count()、sum()、avg()

Object obj = query.uniqueResult()：返回唯一结果

@Test

/**

 * HQL简单查询

 */

public void queryByHQL() {

	Session session = HibernateUtils.getCurrentSession();

	Transaction tx = session.beginTransaction();

	Query query = session.createQuery("select pid,pname,count(*) from Parent group by pid");

	List<Object[]> list = query.list();

	for (Object[] objects : list) {

		System.out.println(Arrays.toString(objects));

	}

	tx.commit();

}

HQL查询（多表）

内连接查询

正常的内连接sql：select * from Children c inner join Parent p

HQL："from Children c inner join c.p "

  • 其中c.p是Children类的Parent属性变量p
  • 返回的是对象数组
package com.qf.entity;

public class Children {

	private Long cid;

	private String cname;

	private Character sex;

	private Parent p;

	....

}
@Test

/**

 * HQL简单查询

 */

public void queryByHQL() {

	Session session = HibernateUtils.getCurrentSession();

	Transaction tx = session.beginTransaction();

	Query query = session.createQuery("from Children c inner join c.p ");

	List<Object[]> list = query.list();

	for (Object[] objects : list) {

		System.out.println(Arrays.toString(objects));

	}

	tx.commit();

}

------------------------------console-------------------------------

[Children [cid=1, cname=张三8, sex=1, p=Parent [pid=1, pname=老张, age=45]], Parent [pid=1, pname=老张, age=45]]

[Children [cid=2, cname=张三0, sex=0, p=Parent [pid=1, pname=老张, age=45]], Parent [pid=1, pname=老张, age=45]]

[Children [cid=3, cname=张三2, sex=1, p=Parent [pid=1, pname=老张, age=45]], Parent [pid=1, pname=老张, age=45]]

...............

[Children [cid=28, cname=王五0, sex=0, p=Parent [pid=3, pname=老王, age=43]], Parent [pid=3, pname=老王, age=43]]

[Children [cid=29, cname=王五8, sex=0, p=Parent [pid=3, pname=老王, age=43]], Parent [pid=3, pname=老王, age=43]]

[Children [cid=30, cname=王五6, sex=1, p=Parent [pid=3, pname=老王, age=43]], Parent [pid=3, pname=老王, age=43]]

迫切内连接

Query query = session.createQuery("from Children c inner join fetch c.p ");

  • 普通内连接HQL的inner join后加上fetch即可
  • 效果：返回的是内连接的左边对象（该例中Parent作为Children属性）
@Test

/**

 * HQL简单查询

 */

public void queryByHQL() {

	Session session = HibernateUtils.getCurrentSession();

	Transaction tx = session.beginTransaction();

	Query query = session.createQuery("from Children c inner join fetch c.p ");

	List<Children> list = query.list();

	for (Children c : list) {

		System.out.println(c);

	}

	tx.commit();

}

------------------------------console-------------------------------

Children [cid=1, cname=张三8, sex=1, p=Parent [pid=1, pname=老张, age=45]]

Children [cid=2, cname=张三0, sex=0, p=Parent [pid=1, pname=老张, age=45]]

Children [cid=3, cname=张三2, sex=1, p=Parent [pid=1, pname=老张, age=45]]

....................

Children [cid=28, cname=王五0, sex=0, p=Parent [pid=3, pname=老王, age=43]]

Children [cid=29, cname=王五8, sex=0, p=Parent [pid=3, pname=老王, age=43]]

Children [cid=30, cname=王五6, sex=1, p=Parent [pid=3, pname=老王, age=43]]

左外连接、右外连接（没有迫切右外连接）以及迫切左外连接使用也类似

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