hibernate 七

                                    
一、集合映射
1、set映射
  关联对象的属性除了外键之外，只有1、2个属性，那么就可以使用set映射
  使用了set标签的element元素，不用创建关联对象就可以实现单向一对多的关联关系
  public class Room implements Serializable{
    private int id;
    private String roomNumber;
    private Set<String> equipments = new HashSet<String>();
    private Set<Image> images = new HashSet<Image>();
  }
  
    <set name="equipments" table="equipment_set">
      <key column="roomid" foreign-key="fk_equip_room_set"/>
      <element column="description" type="string" length="128" not-null="true"/>
    </set>
    <set name="images" table="image_set">
      <key column="roomid" foreign-key="fk_img_room_set"/>
      <composite-element class="Image">
        <property name="path" column="path" type="string" length="50" not-null="true"/>
        <property name="width" column="width" type="integer" />
        <property name="height" column="height" type="integer" />
      </composite-element>   
2、map映射
  非常有用的一种集合映射
  public class Room implements Serializable{
    private int id;
    private String roomNumber;
    private Map<String, String> equipments = new HashMap<String, String>();
    private Map<String, Image> images = new HashMap<String, Image>();
  }
 
    <map name="equipments" table="equipment_map">
      <key column="roomid" foreign-key="fk_equip_room_map"/>
      <map-key column="name" type="string" length="15" />
      <element column="description" type="string" length="128" not-null="true"/>
    </map>
    <map name="images" table="image_map">
      <key column="roomid" foreign-key="fk_img_room_map"/>
      <map-key column="name" type="string" length="15" />
      <composite-element class="Image">
        <property name="path" column="path" type="string" length="50" not-null="true"/>
        <property name="width" column="width" type="integer" />
        <property name="height" column="height" type="integer" />
      </composite-element>
    </map>

二、HQL
1、query
  (1) session.createQuery("from User");                   
      session.createCriteria(User.class);
  
  (2)查询的分页显示
   query.setFirstResult(0);
   query.setMaxResult(10);
    (a) MySql
      select * from student where limite ?,?
    (b) Oracle
      select * from (select row_.* ,rownum rownum_
                  from (select student0_.id as id0_,
                        from student student0_) row)
      where rownum_ <= ?
      and rownum_ > ?                       
  
   (3)在配置文件中的query
   <query name="findByName">
       <![CDATA[      ---  当作字符串看，不做特殊处理
           from User u where u.namelike :name
          
       ]]>   
   </query>
                           
   session.getNamedQuery("findByName").setString("name",name);
  
   from User;   返回对象数组，不可以强转
   select u from User u;  返回对象集合，对于集合中的每个元素都可以强转成User对象
  
2、Dynamic query
   public Collection findStudents(Student student){
         StringBuffer sb = new StringBuffer("select student from Student student where student.id>-1 ");
                                             --- student.id>-1 永远为真，再添加条件的时候都用and即可，避免判断用where还是and
         if(student.getName() != null)
             sb.append("and student.name=:name");
         if(student.getBirthday() != null)
             sb.append("and student.birthday=:birthday");
         if(student.getEmail() != null)
             sb.append("and student.email=:email"); 
         ……                        
   }  
  
3、QBE
   属性多时，效率高，语句少
   忽略主键、version、关系（查询当前对象） 
   与QBC结合，弥补id查询
   Example.excludeProperty("param");  --- 查询时忽略某个属性