Hibernate的查询方式
hibernate的查询方式常见的主要分为三种: HQL, QBC, 以及使用原生SQL查询
HQL查询
• HQL（Hibernate Query Language）提供了丰富灵活的查询方式，使用HQL进行查询也是Hibernate官方推荐使用的查询方式。
• HQL在语法结构上和SQL语句十分的相同，所以可以很快的上手进行使用。使用HQL需要用到Hibernate中的Query对象，该对象专门执行HQL方式的操作。
查询所有示例
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session.beginTransaction();
String hql = "from User"; // from 后跟的是要查询的对象，而不是表
Query query = session.createQuery(hql);
List<User> userList = query.list();
for(User user:userList){
System.out.println(user.getUserName());
}
session.getTransaction().commit();
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带where的查询示例
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session.beginTransaction();
String hql = "from User where userName = 'James'";
Query query = session.createQuery(hql);
List<User> userList = query.list();
for(User user:userList){
System.out.println(user.getUserName());
}
session.getTransaction().commit();
/*
在HQL中where语句中使用的是持久化对象的属性名，如上面示例中的userName。当然在HQL中也可以使用别名
*/
String hql = "from User as u where u.userName = 'James'";
/*
过滤条件
在where语句中还可以使用各种过滤条件，如：=、<>、<、>、>=、<=、between、not between、
in、not in、is、like、and、or等
*/
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获取一个不完整的对象
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session.beginTransaction();
String hql = "select userName from User";
Query query = session.createQuery(hql);
List<Object> nameList = query.list();
for(Object obj:nameList){
String name=(String)obj;
System.out.println(name);
}
session.getTransaction().commit();
// 多个属性的话，需要用object[]接收
session.beginTransaction();
String hql = "select userName,userPwd from User";
Query query = session.createQuery(hql);
List nameList = query.list();
for(Object obj:nameList){
Object[] array = (Object[]) obj; // 转成object[]
System.out.println("name:" + array[0]);
System.out.println("pwd:" + array[1]);
}
session.getTransaction().commit();
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统计和分组查询
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session.beginTransaction();
String hql = "select count(*),max(id) from User";
Query query = session.createQuery(hql);
List nameList = query.list();
for(Object obj:nameList){
Object[] array = (Object[]) obj;
System.out.println("count:" + array[0]);
System.out.println("max:" + array[1]);
}
session.getTransaction().commit();
/*
该条sql语句返回的是单条数据，所以还可以这样写
单列数据用Object，多列数据用Object[]
*/
Object[] object = (Object[]) query.uniqueResult();
System.out.println("count:" + object[0]);
System.out.println("max:" + object[1]);
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更多写法
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select distinct name from Student;
select max(age) from Student;
select count(age),age from Student group by age;
from Student order by age;
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HQL占位符
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session.beginTransaction();
String hql = "from User where userName = ?";
Query query = session.createQuery(hql);
// 索引从0开始
query.setString(0, "James");
List<User> userList = query.list();
for(User user:userList){
System.out.println(user.getUserName());
}
session.getTransaction().commit();
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HQL引用占位符
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session.beginTransaction();
String hql = "from User where userName = :name";
Query query = session.createQuery(hql);
query.setParameter("name", "James");
List<User> userList = query.list();
for(User user:userList){
System.out.println(user.getUserName());
}
session.getTransaction().commit();
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HQL分页
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session.beginTransaction();
String hql = "from User";
Query query = session.createQuery(hql);
query.setFirstResult(0);
query.setMaxResults(2);
List<User> userList = query.list();
for(User user:userList){
System.out.println(user.getUserName());
}
session.getTransaction().commit();
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QBC（Query By Criteria）查询
• Criteria对象提供了一种面向对象的方式查询数据库。Criteria对象需要使用Session对象来获得。
• 一个Criteria对象表示对一个持久化类的查询。
查询所有
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session.beginTransaction();
Criteria c = session.createCriteria(User.class);
List<User> userList = c.list();
for(User user:userList){
System.out.println(user.getUserName());
}
session.getTransaction().commit();
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where
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session.beginTransaction();
Criteria c = session.createCriteria(User.class);
c.add(Restrictions.eq("userName", "James"));
List<User> userList = c.list();
for(User user:userList){
System.out.println(user.getUserName());
}
session.getTransaction().commit();
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Restrictions对象
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方法名称 对应SQL中的表达式
----------------------------------------------------------
Restrictions.eq field = value
Restrictions.gt field > value
Restrictions.lt field < value
Restrictions.ge field >= value
Restrictions.le field <= value
Restrictions.between field between value1 and value2
Restrictions.in field in(…)
Restrictions.and and
Restrictions.or or
Restrictions.like field like value
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示例
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Criteria c = session.createCriteria(User.class);
c.add(Restrictions.like("userName", "J"));
c.add(Restrictions.eq("id", 120));
c.add(Restrictions.or(Restrictions.eq("userName", "James"),
Restrictions.eq("userName", "Alex")));
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获取唯一记录
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session.beginTransaction();
Criteria c = session.createCriteria(User.class);
c.add(Restrictions.eq("id", 120));
User user = (User) c.uniqueResult();
System.out.println(user.getUserName());
session.getTransaction().commit();
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分页
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Criteria c = session.createCriteria(User.class);
c.setFirstResult(0);
c.setMaxResults(5);
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分组与统计
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session.beginTransaction();
Criteria c = session.createCriteria(User.class);
c.setProjection(Projections.sum("id"));
Object obj = c.uniqueResult();
System.out.println(obj);
session.getTransaction().commit();
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Projections对象
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方法名称 描述
-------------------------------------------------------
Projections.sum 等于SQL中聚合函数sum
Projections.avg 等于SQL中聚合函数avg
Projections.count 等于SQL中聚合函数count
Projections .distinct 去除重复记录
Projections.max 等于SQL中聚合函数max
Projections.min 等于SQL中聚合函数min
Projections .groupProperty 对指定的属性进行分组查询
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多个统计与分组
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session.beginTransaction();
Criteria c = session.createCriteria(User.class);
ProjectionList projectionList = Projections.projectionList();
projectionList.add(Projections.sum("id"));
projectionList.add(Projections.min("id"));
c.setProjection(projectionList);
// 和HQL一样，单列用Object，多列用Object[]
Object[] obj = (Object[]) c.uniqueResult();
System.out.println("sum:" + obj[0]);
System.out.println("min:" + obj[1]);
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排序
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Criteria c = session.createCriteria(User.class);
c.addOrder(Order.desc("id"));
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原生SQL查询：
示例
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session.beginTransaction();
String sql = "select id,username,userpwd from t_user";
List list = session.createSQLQuery(sql).list();
for(Object item : list){
Object[] rows = (Object[]) item;
System.out.println("id:" + rows[0] + "username:"
+ rows[1] + "userpwd:" + rows[2]);
}
session.getTransaction().commit();
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addEntity()示例
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session.beginTransaction();
String sql = "select id,username,userpwd from t_user";
// addEntity()可以告诉Hibernate你想要封装成对象的类型，然后自动为你封装
SQLQuery query = session.createSQLQuery(sql).addEntity(User.class);
List<User> list = query.list();
for(User user : list){
System.out.println(user.getUserName());
}
session.getTransaction().commit();
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uniqueResult示例
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session.beginTransaction();
String sql = "select id,username,userpwd from t_user where id = 2";
SQLQuery query = session.createSQLQuery(sql).addEntity(User.class);
User user = (User) query.uniqueResult();
System.out.println(user.getUserName());
session.getTransaction().commit();
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