Hibernate 读取部分列
在普通的sql中，为了选取某些列，其实是很简单的，就select 列名就可以了，但在hibernate中，一般
都是sql="from user ....";但这样其实某些时候，效率还是很低的，在hibernate做的时候，要这样了，如下：
String hql="select new map(t.title as title,t.id as id,t.link as link,t.linktitle as linktitle) from Article as t? where t.category="+categoryid+" order by t.uploadtime desc";
?
Query query= getSessionFactory().getCurrentSession().createQuery(hql);
??query.setFirstResult((pageNo - 1) * pageSize);
??query.setMaxResults(pageSize);
???? List<Map> list = query.list();???
???????? for(Map article : list)
???????? {
???????? ? String title = (String)article.get("title");???
???????? ?System.out.println("文章标题是"+title);
???????? }
?
? 下面再介绍下相关的知识：
1? 查询其中几个字段?
?? ??String?hql?=?"?select?name,passwd?from?Users";??????
????????Query?query?=?session.createQuery(hql);??????
????????//默认查询出来的list里存放的是一个Object数组??????
????????List<Object[]>?list?=?query.list();??????
????????for(Object[]?object?:?list){??????
????????????String?name?=?(String)object[0];??????
????????????String?passwd?=?(String)object[1];??????
??????????????????
????????????System.out.println(name?+?"?:?"?+?passwd);??????
????????}??????
2 修改默认查询结果(query.list())不以Object[]数组形式返回，以List形式返回
??//查询其中几个字段,添加new?list(),注意list里的l是小写的。也不需要导入包，这样通过query.list()出来的list里存放的不再是默认的Object数组了，而是List集合了???
????????String?hql?=?"?select?new?list(name,passwd)?from?Users";???
????????Query?query?=?session.createQuery(hql);???
????????//默认查询出来的list里存放的是一个Object数组，但是在这里list里存放的不再是默认的Object数组了，而是List集合了???
????????List<List>?list?=?query.list();???
????????for(List?user?:?list){???
????????????String?name?=?(String)user.get(0);???
????????????String?passwd?=?(String)user.get(1);???
???????????????
????????????System.out.println(name?+?"?:?"?+?passwd);???
????????}???
? 要注意的是，假如这时有些列是为空的话，则String?hql?=?"?select?new?list(name,passwd)?from?Users";???是会报错的，要用下面的方法：
?
3?//查询其中几个字段,添加new?map(),注意map里的m是小写的。也不需要导入包，这样通过query.list()出来的list里存放的不再是默认的Object数组了，而是map集合了??????
????????String?hql?=?"?select?new?map(name,passwd)?from?Users";??????
????????Query?query?=?session.createQuery(hql);??????
????????//默认查询出来的list里存放的是一个Object数组，但是在这里list里存放的不再是默认的Object数组了，而是Map集合了??????
????????List<Map>?list?=?query.list();??????
????????for(Map?user?:?list){??????
????????????//一条记录里所有的字段值都是map里的一个元素,key是字符串0,1,2,3
.，value是字段值??????
????????????//如果将hql改为：String?hql?=?"?select?new?map(name?as?username,passwd?as?password)?from?Users";,那么key将不是字符串0,1,2了，而是"username","password"了??????
????????????String?name?=?(String)user.get("0");//get("0");是get(key),注意:0,1,2是字符串，而不是整形??????
????????????String?passwd?=?(String)user.get("1");??????
??????????????????
????????????System.out.println(name?+?"?:?"?+?passwd);??????
????????}??????
4 修改默认查询结果(query.list())不以Object[]数组形式返回，以自定义类型返回
???package?com.domain;???
??
public?class?MyUser?{???
??
????private?String?username;???
????private?String?password;???
//因为：String?hql?=?"?select?new??com.domain.MyUser(name,passwd)?from?Users";所以必须要有接受2个参数的构造函数???
????public?MyUser(String?username,String?password){???
????????this.username?=?username;???
????????this.password?=?password;???
????}???
???????
????public?String?getUsername()?{???
????????return?username;???
????}???
????public?void?setUsername(String?username)?{???
????????this.username?=?username;???
????}???
????public?String?getPassword()?{???
????????return?password;???
????}???
????public?void?setPassword(String?password)?{???
????????this.password?=?password;???
????}???
???????
???????
}??
//通过query.list()出来的list里存放的不再是默认的Object数组了,而是自定义的类MyUser,必须加包名，String?hql?=?"from?Users";中的Users类也是必须加包名的，但是因为再Users.hbm.xml里<hibernate-mapping?auto-import="true">?auto-import默认值为true（所以auto-import属性也可以不写）,自动导入了???
????????String?hql?=?"?select?new??com.domain.MyUser(name,passwd)?from?Users";???
????????Query?query?=?session.createQuery(hql);???
????????//默认查询出来的list里存放的是一个Object数组，但是在这里list里存放的不再是默认的Object数组了，而是MyUser对象了???
????????List<MyUser>?myUsers?=?query.list();???
????????for(MyUser?myUser?:?myUsers){???
????????????String?name?=?myUser.getUsername();???
????????????String?passwd?=?myUser.getPassword();???
????????????System.out.println(name?+?"?:?"?+?passwd);???
????????}???
5 条件查询
??//条件查询，参数索引值从0开始，索引位置。通过setString,setParameter设置参数???
????????String?hql?=?"from?Users?where?name=??and?passwd=?";???
????????Query?query?=?session.createQuery(hql);???
????????//第1种方式???
//??????query.setString(0,?"name1");???
//??????query.setString(1,?"password1");???
????????//第2种方式???
????????query.setParameter(0,?"name1",Hibernate.STRING);???
????????query.setParameter(1,?"password1",Hibernate.STRING);???
????????List<Users>?list?=?query.list();???
????????for(Users?users?:?list){???
????????????System.out.println(users.getId());???
????????}???
//条件查询，自定义索引名(参数名):username,:password.通过setString,setParameter设置参数???
????????String?hql?=?"from?Users?where?name=:username?and?passwd=:password";???
????????Query?query?=?session.createQuery(hql);???
????????//第1种方式???
//??????query.setString("username",?"name1");???
//??????query.setString("password",?"password1");???
????????//第2种方式,第3个参数确定类型???
????????query.setParameter("username",?"name1",Hibernate.STRING);???
????????query.setParameter("password",?"password1",Hibernate.STRING);???
????????List<Users>?list?=?query.list();???
????????for(Users?users?:?list){???
????????????System.out.println(users.getId());???
????????}??
?
//条件查询,通过setProperties设置参数???
????????String?hql?=?"from?Users?where?name=:username?and?passwd=:password";???
????????Query?query?=?session.createQuery(hql);???
????????//MyUser类的2个属性必须和:username和:password对应???
????????MyUser?myUser?=?new?MyUser("name1","password1");???
????????query.setProperties(myUser);???
????????List<Users>?list?=?query.list();???
????????for(Users?users?:?list){???
????????????System.out.println(users.getId());???
????????}??
???????
假如觉得还是想用回原生SQL的话，可以这样：
? Query query = getSessionFactory().getCurrentSession().createSQLQuery(hql).addScalar("title",Hibernate.STRING).addScalar("id",Hibernate.INTEGER).setResultTransformer(Transformers.aliasToBean(Article.class));
就是要注意的话，这里的createSQLQuery里面跟的是原生的sql，要读什么属性，就要不断加addScalar，个人觉得这个方式十分麻烦，呵呵
