6.HQL入门

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在hql中关键字不区分大小写，但是属性和类名区分大小写

1、简单属性查询【重要】

* 单一属性查询，返回结果集属性列表，元素类型和实体类中相应的属性类型一致

List students = session.createQuery("select name from Student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

String name = (String)iter.next();

System.out.println(name);

}

* 多个属性查询,返回的集合元素是对象数组,数组元素的类型和对应的属性在实体类中的类型一致

数组的长度取决与select中属性的个数

List students = session.createQuery("select id, name from Student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Object[] obj = (Object[])iter.next();

System.out.println(obj[0] + "," + obj[1]);

}

* 如果认为返回数组不够对象化，可以采用hql动态实例化Student对象

List students = session.createQuery("select new Student(id, name) from Student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getId() + "," + student.getName());

}

*使用别名

List students = session.createQuery("select s.id, s.name from Student s").list();

List students = session.createQuery("select s.id, s.name from Student as s").list();

2、实体对象查询【重要】

*实体对象查询

//返回Student对象的集合

//可以忽略select

List students = session.createQuery("from Student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

//返回Student对象的集合

//可以忽略select,表可以使用别名

List students = session.createQuery("from Student s").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

//返回Student对象的集合

//可以忽略select,表可以使用as命名别名

List students = session.createQuery("from Student as s").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

//返回Student对象的集合

//使用select查询实体对象，必须采用别名

List students = session.createQuery("select s from Student as s").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

//不支持select * from .....这样的查询语句

List students = session.createQuery("select * from Student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

* N + 1问题，在默认情况下，使用query.iterate查询，有可以能出现N+1问题

所谓的N+1是在查询的时候发出了N+1条sql语句

1: 首先发出一条查询对象id列表的sql

N: 根据id列表到缓存中查询，如果缓存中不存在与之匹配的数据，那么会根据id发出相应的sql语句

* list和iterate的区别？

* list每次都会发出sql语句，list会向缓存中放入数据，而不利用缓存中的数据

* iterate：在默认情况下iterate利用缓存数据，但如果缓存中不存在数据有可以能出现N+1问题

/**

* 采用list查询发出一条查询语句，取得Student对象数据、

* Hibernate: select student0_.id as id1_, student0_.name as name1_,

* student0_.createTime as createTime1_, student0_.classesid as classesid1_

* from t_student student0_

List students = session.createQuery("from Student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

/**

* 出现N+1问题

* 1：发出查询id列表的sql

* Hibernate: select student0_.id as col_0_0_ from t_student student0_

* N：在依次发出根据id查询Student对象的sql

* Hibernate: select student0_.id as id1_0_, student0_.name as name1_0_,

* student0_.createTime as createTime1_0_, student0_.classesid as classesid1_0_

* from t_student student0_ where student0_.id=?

Iterator iter = session.createQuery("from Student").iterate();

while(iter.hasNext()) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

/**

* 不会出现N+1问题

* 因为list操作已经将Student对象放到了一级缓存中，所以再次使用iterate操作的时候

* 它首先发出一条查询id列表的sql，在根据id到缓存中去数据，只有在缓存中找不到相应的

* 数据时，才会发出sql到数据库中查询

List students = session.createQuery("from Student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

System.out.println("---------------------------------------------");

Iterator iter = session.createQuery("from Student").iterate();

while(iter.hasNext()) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

/**

* 两次list操作

* 在默认情况下list每次都会向数据库发出查询对象的sql，除非配置查询缓存，所以第二次的list操作

* 虽然在一级缓存中已经有了对象数据，但list默认情况下不会利用缓存，而再次发出sql

* 默认情况下，list会向缓存中放入数据，但不会利用数据

List students = session.createQuery("from Student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

System.out.println("---------------------------------------------");

students = session.createQuery("from Student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

3、条件查询【重要】

* 可以采用拼字符串的方式传递参数

//可以拼字符串

List students = session.createQuery("select s.id, s.name from Student s where s.name like '%1%'").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Object[] obj = (Object[])iter.next();

System.out.println(obj[0] + "," + obj[1]);

}

* 可以采用？来传递参数（索引从0开始）

//可以使用？方式传递参数

//参数的索引从0开始

//传递的参数值，不用单引号引起来

//注意方法链编程

List students = session.createQuery("select s.id, s.name from Student s where s.name like ?")

.setParameter(0, "%1%")

.list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Object[] obj = (Object[])iter.next();

System.out.println(obj[0] + "," + obj[1]);

}

* 可以采用 :参数名来传递参数

//使用：参数名称的方式传递参数值

List students = session.createQuery("select s.id, s.name from Student s where s.name like :myname")

.setParameter("myname", "%1%")

.list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Object[] obj = (Object[])iter.next();

System.out.println(obj[0] + "," + obj[1]);

}

//使用：参数名称的方式传递参数值

List students = session.createQuery("select s.id, s.name from Student s where s.name like :myname and s.id=:myid")

.setParameter("myname", "%1%")

.setParameter("myid", 12)

.list();

* 如果传递多个参数，可以采用setParamterList方法

//支持in，需要使用setParameterList进行参数传递

List students = session.createQuery("select s.id, s.name from Student s where s.id in(:myids)")

.setParameterList("myids", new Object[]{1, 2, 3, 4, 5})

.list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Object[] obj = (Object[])iter.next();

System.out.println(obj[0] + "," + obj[1]);

}

* 在hql中可以使用数据库的函数，如：date_format

//查询2008年2月创建的学生

List students = session.createQuery("select s.id, s.name from Student s where date_format(s.createTime, '%Y-%m')=?")

.setParameter(0, "2008-02")

.list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Object[] obj = (Object[])iter.next();

System.out.println(obj[0] + "," + obj[1]);

}

//查询2008-01-10到2008-02-15创建的学生

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

List students = session.createQuery("select s.id, s.name from Student s where s.createTime between ? and ?")

.setParameter(0, sdf.parse("2008-01-10 00:00:00"))

.setParameter(1, sdf.parse("2008-02-15 23:59:59"))

.list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Object[] obj = (Object[])iter.next();

System.out.println(obj[0] + "," + obj[1]);

}

4、hibernate也支持直接使用sql进行查询

List students = session.createSQLQuery("select * from t_student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Object[] obj = (Object[])iter.next();

System.out.println(obj[0] + "," + obj[1]);

}

5、外置命名查询

* 在映射文件中采用<query>标签来定义hql

<![CDATA[

SELECT s FROM Student s where s.id<?

]]>

</query>

* 在程序中采用session.getNamedQuery()方法得到hql查询串

List students = session.getNamedQuery("searchStudents")

.setParameter(0, 10)

.list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

6、查询过滤器

* 在映射文件中定义过滤器参数

<filter-def name="filtertest">

<filter-param name="myid" type="integer"/>

</filter-def>

* 在类的映射中使用这些参数

</id>

<many-to-one name="classes" column="classesid"/>

</class>

* 在程序中启用过滤器

session.enableFilter("filtertest").setParameter("myid", 10);

List students = session.createQuery("from Student").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

7、分页查询【重要】

* setFirstResult()，从0开始

* setMaxResults,每页显示多少条数据

List students = session.createQuery("from Student")

.setFirstResult(1)

.setMaxResults(2)

.list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Student student = (Student)iter.next();

System.out.println(student.getName());

}

8、对象导航查询，在hql中采用 . 进行导航【重要】

List students = session.createQuery("select s.name from Student s where s.classes.name like '%1%'").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

String name = (String)iter.next();

System.out.println(name);

}

9、连接查询【重要】

* 内连

//List students = session.createQuery("select c.name, s.name from Student s join s.classes c").list();

List students = session.createQuery("select c.name, s.name from Student s inner join s.classes c").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Object[] obj = (Object[])iter.next();

System.out.println(obj[0] + "," + obj[1]);

}

* 外连接（左连接/右连接）

List students = session.createQuery("select c.name, s.name from Classes c left join c.students s").list();//左外连接

List students = session.createQuery("select c.name, s.name from Classes c right join c.students s").list();//右外连接

10、统计查询【重要】

Long count = (Long)session.createQuery("select count(*) from Student").uniqueResult();

System.out.println(count);

List students =session.createQuery("select c.name, count(s) from Student s join s.classes c " +

"group by c.name order by c.name").list();

for (Iterator iter=students.iterator(); iter.hasNext();) {

Object[] obj = (Object[])iter.next();

System.out.println(obj[0] + ", " + obj[1]);

}

11、DML风格的操作（尽量少用，因为和缓存不同步）

session.createQuery("update Student s set s.name=? where s.id < ?")

.setParameter(0, "李四")

.setParameter(1, 5)

.executeUpdate();