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HQL(Hibernate Query Language)跟我们以前用的SQL有很多相似之处,但是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。HQL的关键字不区分大小写,类名与属性名是区分大小写的。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。
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