HQL: Hibernate Query Language.
HQL特点： 1，与SQL相似，SQL中的语法基本上都可以直接使用。 2，SQL查询的是表和表中的列；HQL查询的是对象与对象中的属性。 3，HQL的关键字不区分大小写，类名与属性名是区分大小写的。 4，SELECT可以省略。
1，简单的查询，Employee为实体名而不是数据库中的表名(面向对象特性)
hql = "FROM Employee"; hql = "FROM Employee as e"; // 使用别名 hql = "FROM Employee e"; // 使用别名，as关键字可省略2，带上过滤条件的（可以使用别名）：Where
hql = "FROM Employee WHERE id<10"; hql = "FROM Employee e WHERE e.id<10"; hql = "FROM Employee e WHERE e.id<10 AND e.id>5";3，带上排序条件的：Order By
hql = "FROM Employee e WHERE e.id<10 ORDER BY e.name"; hql = "FROM Employee e WHERE e.id<10 ORDER BY e.name DESC"; hql = "FROM Employee e WHERE e.id<10 ORDER BY e.name DESC, id ASC";4，指定select子句（不可以使用select *）
hql = "SELECT e FROM Employee e"; // 相当于"FROM Employee e" hql = "SELECT e.name FROM Employee e"; // 只查询一个列，返回的集合的元素类型就是这个属性的类型 hql = "SELECT e.id,e.name FROM Employee e"; // 查询多个列，返回的集合的元素类型是Object数组 hql = "SELECT new Employee(e.id,e.name) FROM Employee e"; // 可以使用new语法，指定把查询出的部分属性封装到对象中5，执行查询，获得结果（list、uniqueResult、分页 ）
Query query = session.createQuery("FROM Employee e WHERE id<3"); query.setFirstResult(0); query.setMaxResults(10); // 等同于 limit 0,10 //两种查询结果list、uniqueResult List list = query.list(); // 查询的结果是一个List集合 Employee employee = (Employee) query.uniqueResult();// 查询的结果是唯一的一个结果，当结果有多个，就会抛异常6，方法链
List list = session.createQuery( "FROM Employee e") .setFirstResult(0) .setMaxResults(10) .list();7，聚集函数：count(), max(), min(), avg(), sum()
hql = "SELECT COUNT(*) FROM Employee"; // 返回的结果是Long型的 hql = "SELECT min(id) FROM Employee"; // 返回的结果是id属性的类型8，分组: Group By … Having
hql = "SELECT e.name,COUNT(e.id) FROM Employee e GROUP BY e.name"; hql = "SELECT e.name,COUNT(e.id) FROM Employee e GROUP BY e.name HAVING count(e.id)>1"; hql = "SELECT e.name,COUNT(e.id) FROM Employee e WHERE id<9 GROUP BY e.name HAVING count(e.id)>1"; hql = "SELECT e.name,COUNT(e.id) " + "FROM Employee e " + "WHERE id<9 " + "GROUP BY e.name " + "HAVING count(e.id)>1 " + "ORDER BY count(e.id) ASC"; hql = "SELECT e.name,COUNT(e.id) AS c " + "FROM Employee e " + "WHERE id<9 " + "GROUP BY e.name " + "HAVING count(e.id)>1 " + // 在having子句中不能使用列别名 "ORDER BY c ASC"; // 在orderby子句中可以使用列别名9，连接查询 / HQL是面向对象的查询
// 内连接（inner关键字可以省略） hql = "SELECT e.id,e.name,d.name FROM Employee e JOIN e.department d"; hql = "SELECT e.id,e.name,d.name FROM Employee e INNER JOIN e.department d"; // 左外连接（outer关键字可以省略） hql = "SELECT e.id,e.name,d.name FROM Employee e LEFT OUTER JOIN e.department d"; // 右外连接（outer关键字可以省略） hql = "SELECT e.id,e.name,d.name FROM Employee e RIGHT JOIN e.department d"; // 可以使用更方便的方法 hql = "SELECT e.id,e.name,e.department.name FROM Employee e";10，查询时使用参数
// 方式一：使用'?'占位 hql = "FROM Employee e WHERE id BETWEEN ? AND ?"; List list2 = session.createQuery(hql) .setParameter(0, 5)// 设置参数，第1个参数的索引为0。 .setParameter(1, 15) .list(); // 方式二：使用变量名 hql = "FROM Employee e WHERE id BETWEEN :idMin AND :idMax"; List list3 = session.createQuery(hql) .setParameter("idMax", 15) .setParameter("idMin", 5) .list(); // 当参数是集合时，一定要使用setParameterList()设置参数值 hql = "FROM Employee e WHERE id IN (:ids)"; List list4 = session.createQuery(hql) .setParameterList("ids", new Object[] { 1, 2, 3, 5, 8, 100 }) .list();11，update与delete，不会通知Session缓存
// >> Update int result = session.createQuery( "UPDATE Employee e SET e.name=? WHERE id>15") .setParameter(0, "无名氏") .executeUpdate(); // 返回int型的结果，表示影响了多少行。 // >> Delete int result1 = session.createQuery( "DELETE FROM Employee e WHERE id>15") .executeUpdate(); 返回int型的结果，表示影响了多少行。本文是转载的，出处:http://blog.csdn.net/v123411739/article/details/28644007
