Hibernate HQL 级联注意事项

Hibernate HQL 级联注意事项

 

Hibernate级联查询(HQL)

拿雇员(Employee)与部门(Department)来举例子说明

1.1    数据库模型

1.1.1          Employee表

DROP TABLE IF EXISTS employee; CREATE TABLE employee (  id int(11) NOT NULL auto_increment,  empname varchar(255) default NULL,  empage int(11) default NULL,  context varchar(255) default NULL,  deptid int(11) default NULL,  PRIMARY KEY (id),  KEY FK4722E6AEAD9148B6 (deptid) ) ENGINE=InnoDB DEFAULT CHARSET=gbk;

1.1.2          Department表

DROP TABLE IF EXISTS department; CREATE TABLE department (  id int(11) NOT NULL auto_increment,  deptname varchar(255) default NULL,  PRIMARY KEY (id) ) ENGINE=InnoDB DEFAULT CHARSET=gbk;

1.2    Java类

publicclass Employee {     private Integer id;     private String empname;     private Integer empage;     private String context;     private Integer deptid;     private Department department=new Department();       //此处省略set()、get()方法 }

publicclass Department {     private Integer id;     private String deptname;     private Set<Employee> employeeSet=new HashSet<Employee>(); //此处省略set()、get()方法 }

1.3    映射文件

<hibernate-mapping package="com.dl.bean">     <class name="Employee" table="employee">          <id name="id" >              <generator class="native"/>          </id>          <property name="empname"/>          <property name="empage"/>          <property name="context" />          <property name="deptid" insert="false" update="false"/>          <many-to-one name="department" column="deptid" cascade="all" class="Department" />     </class>     </hibernate-mapping>

<hibernate-mapping package="com.dl.bean">     <class name="Department" table="department">          <id name="id" >              <generator class="native"/>          </id>          <property name="deptname" />              <set name="employeeSet">                  <key column="deptid"></key>                  <one-to-many class="Employee"/>              </set>     </class>     </hibernate-mapping>

1.4    在写Hibernate级联查询的HQL语句时，与传统的JDBC方式有着本质的区别

1.4.1          JDBC级联写法

SELECT * FROM employee INNER JOIN department ON employee.deptid= department.id

1.4.2          HQL级联写法

select employee.id, employee.empname, employee.empage, employee.context, employee.department.deptname from Employee employee inner join employee.department (此处与前文中Employee类中的属性名一致) order by employee.id desc

注意红色字体部分，在传统的SQL语句中inner join后面跟级联的表，但在HQL中inner join后面跟能反映出级联关系的字段属性(与pojo中的名字必须高度保持一致),当存在级联时,Hibernate查询结果为一对象数组,其数组的长度取决于select与from之间字段的个数。

1.5    注意分析下面三条语句看有什么不同

SQLQuery方式 sqlquery=session.createSQLQuery("select employee.id,employee.empname,employee.empage,employee.context,department.deptname from employee inner join department on employee.deptid=department.id");

Query方式: session.createQuery("select employee.id,employee.empname,employee.empage,employee.context,employee.department.deptname from Employee employee inner join employee.department order by employee.id desc");

Query方式: Query query=session.createQuery("select employee.empname, employee.department from Employee as employee order by employee.id desc");

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