SQL中，查询语句的编写是很重要的部分。Hibernate中，可以使用面向对象的HQL语句，SQL操作的是数据表，列等，HQL操作的就是对象，属性等，HQL的语法和SQL很像，HQL关键字不区分大小写，但是类名，属性名等是区分大小写的。
这里使用的例子是上一节中的老师-同学双向N-N。
Student.java
@Entity
@Table(name = "t_student")
public class Student {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "student_id")
private Long studentId;
@Column(name = "name")
private String studentName;
@ManyToMany(mappedBy = "studentSet")
private Set<Teacher> teacherSet = new HashSet<>();
//省略构造函数，get/set方法略
}
Teacher.java
@Entity
@Table(name = "t_teacher")
public class Teacher {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "teacher_id")
private Long teacherId;
@Column(name = "name")
private String teacherName;
@ManyToMany
@JoinTable(name = "teacher_student",
joinColumns = {@JoinColumn(name = "teacher_id")},
inverseJoinColumns = {@JoinColumn(name = "student_id")})
private Set<Student> studentSet = new HashSet<>();
//省略构造函数，get/set方法略
}
Session session = sessionFactory.openSession();
Transaction transaction = session.beginTransaction();
List<Teacher> teacherList = session.createQuery("from Teacher t where t.teacherName = :teacherName")
.setString("teacherName", "t1")
.list();
for(Teacher t : teacherList) {
System.out.println(t);
}
transaction.commit();
session.close();
我们发现，HQL里连SELECT都不用，实际上按照SQL的习惯写成select t from ...也是可以的。查询指定的是对象，而不是某张数据表。
除此之外，注意HQL中占位符的写法，是冒号加上占位符名。
List<Student> studentList = session.createQuery("select s from Student s inner join s.teacherSet t where t.teacherName = :teacherName")
.setString("teacherName", "t1")
.list();
Session session = sessionFactory.openSession();
Transaction transaction = session.beginTransaction();
List<Object[]> studentAttrList = session.createQuery("select s.studentId, s.studentName from Student s inner join s.teacherSet t where t.teacherName = :teacherName")
.setString("teacherName", "t1")
.list();
for(Object[] arr : studentAttrList) {
System.out.println(Arrays.toString(arr));
}
transaction.commit();
session.close();
HQL和SQL一样，深入研究是有点复杂的，这里就不多举例了。此外，我们还要知道，Hibernate也是支持使用SQL语句的，如果一些奇特的功能HQL无法完成，完全可以使用SQL进行代替。更多有关HQL的内容，可以参考文档。