3.[精通Hibernate笔记]Hibernate检索方式

1.Hibernate查询方式

Hibernate提供了以下几种检索对象的方式：

HQL检索方式

HQL（Hibernate Q是面向对象的查询语言，它具有以下功能：

在查询语句中设定各种查询条件；

支持投影查询，仅查询出对象的部分属性；

支持分页查询；

支持连接查询；

支持分组查询，允许使用having和group by关键字；

提供内置聚合函数，如sun()、min()和max()；

能够调用用户定义的SQL函数；

支持子查询，即嵌入式查询；

支持动态绑定参数。

Session类的find()方法及Query接口都支持HQL检索方式，前者只是执行一些简单HQL查询语句，它不具有动态绑定参数的功能，而在新Hibernate版本中，有可能淘汰find()方法；而Query接口才是真正的HQL查询接口。

Query query = session.createQuery("from User as u where u.name = :userName and u.age = :userAge");
query.setString("userName", "Tom");
query.setInteger("userAge", 21);
List result = query.list();

QBC检索方式

采用HQL检索方式时，在应用程序汇总需要定义基于字符串形式的HQL查询语句。QBC (Query By Criteria) API提供检索对象的另一种方式，它主要由Criteria接口、Criterion接口和Expression类组成，它支持在运行时动态生成查询语句。

Criteria criteria = session.createCriteria(User.class);
Criterion criterion1 = Expression.like("name", "T%");
Criterion criterion2 = Expression.eq("age", new Integer(21));
criteria = criteria.add(criterion1);
criteria = criteria.add(criterion2);
List result = criteria.list();

SQL检索方式

采用HQL或QBC检索方式时，Hibernate会生成标准的SQL查询语句，适用于所有的数据库平台，因袭这两种方式都是跨平台的。

有的应用程序可能需要根据底层数据库的SQL方言，来生成一些特殊的查询语句。在这种情况下，Hibernate提供SQL检索方式。

Query query = session.createSQLQuery("select {u.*} from User u where u.NAME like : userName and u.AGE = :userAge");
query.setString("userName", "Tom");
query.setInteger("userAge", 21);
List result = query.list();

使用别名

from User as u where u.name = :username

as关键字用于设定别名，也可以将as关键字省略：

from User u where u.name = :username

对查询结果排序

Query query = session.createQuery("from User as u order by u.name");

分页查询

Query query = session.createQuery("from User as u order by u.name asc");
query.setFirstResult(0);
query.setMaxResults(10);
List result = query.list();

查询单个对象

User user = (User)session.createQuery("from User as u order by u.name asc").setMaxResults(1).uniqueResult();

在HQL查询语句中绑定参数

Query query = session.createQuery("from User as u where u.name = '" + name + "' and u.age = " + age);

按参数名绑定

Query query = session.createQuery("from User as u where u.name = :userName and u.age = :userAge");

调用Query的setXXX()方法来绑定参数：

query.setString("userName", name);
query.setInteger("userAge", age);

按参数位置绑定

Query query = session.createQuery("from User as u where u.name = ? and u.age = ?");

调用Query的setXXX()方法来绑定参数：

query.setString(0, name);
query.setInteger(1, age);

2.设定查询条件

比较运算

session.createQuery("from User as u where u.age > 18");
session.createQuery("from User as u where u.age <> 18");
session.createQuery("from User as u where u.name is null");
session.createQuery("from User as u where lower(u.name) = 'tom'");
session.createQuery("from User as u where upper(u.name) = 'TOM'");
session.createQuery("from User as u where u.age*4-100>10");

范围运算

session.createQuery("from User as u where u.name in('Tom','Jack')");
session.createQuery("from User as u where u.age between 18 and 25");
session.createQuery("from User as u where u.age not between 18 and 25");

字符串模式匹配

session.createQuery("from User as u where u.name like 'T%'");
session.createQuery("from User as u where u.name like '%om%'");
session.createQuery("from User as u where u.name like 'T__'");

逻辑运算

session.createQuery("from User as u where u.name like 'T%' and u.name like '%m%'");
session.createQuery("from User as u where (u.name like 'T%' or u.name like '%m%')");

3.连接查询

检索策略

List result = session.createQuery("from User as u where u.name like 'T%'").list();
for (Iterator it = result.iterator(); it.hasNext();) {
    User user = (User)it.next();
}

左外连接

List result = session.createQuery("from User as u left join u.roles where u.name like 'T%'").list();
for (Iterator pairs = result.iterator(); pairs.hasNext();) {
    Object[] pair = (Object[])pairs.next();
    User user = (User)pair[0];
    Role role = (Role)pair[1];
}

内连接

List result = session.createQuery("from User as u inner join u.roles where u.name like 'T%'").list();
for (Iterator pairs = result.iterator(); pairs.hasNext();) {
    Object[] pair = (Object[])pairs.next();
    User user = (User)pair[0];
    Role role = (Role)pair[1];
}

右外连接

List result = session.createQuery("from User as u right outer join u.roles where u.name like 'T%'").list();
for (Iterator pairs = result.iterator(); pairs.hasNext();) {
    Object[] pair = (Object[])pairs.next();
    User user = (User)pair[0];
    Role role = (Role)pair[1];
}

4.报表查询

投影查询

投影查询是查询结果仅包含部分实体或实体的部分属性。投影是通过select关键字来实现的。

List result = session.createQuery("select u.id,u.name,r.rolename from User u join u.roles r where u.name like 'T%'").list();

使用聚集函数

Integer count = (Integer)session.createQuery("select count(*) from User u").uniqueResult();
Float age = (Float)session.createQuery("select avg(u.age) from User u").uniqueResult();
Object[] os = (Object[])session.createQuery("select max(u.age),min(u.age) from User u").uniqueResult();
Integer count = (Integer)session.createQuery("select count(sistinct u.name) from User u").uniqueResult();

分组查询

List result = session.createQuery("select u.name,count(u) from User u group by u.name").list();
List result = session.createQuery("select u.name,count(u) from User u group by u.name having (count(u)>1").list();

5.高级查询技巧

动态查询

public List findUsers(String name, int age) {
    StringBuffer hqlStr = new StringBuffer("from User u");
    if (name != null)
        hqlStr.append(" where lower(c.name) like :name");
    if (age != 0 && name != null)
        hqlStr.append(" and u.age = :age");
    if (age != 0 && name != null)
        hqlStr.append(" where u.age = :age");
    Query query = session.createQuery(hqlStr.toString());
    if (name != null)
        query.setString("name", name.toLowerCase());
    if (age != 0)
        query.setInteger("age", age);
    return query.list();
}

子查询

List result = session.createQuery("from User u where 1<(select count(o) from u.roles r)").list();

说明：笔记内容摘自《精通Hibernate：Java对象持久化技术详解》

关联：整理了一些Java软件工程师的基础知识点