hibernate 查询与连接
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create database schoolproject;
use schoolproject;
drop table if exists certificate;
CREATE TABLE certificate (
id varchar(100) NOT NULL default '',
`describe` varchar(100) default '',
PRIMARY KEY (id)
) ENGINE = InnoDB
CHARACTER SET utf8 COLLATE utf8_general_ci;
--
-- Dumping data for table 'certificate'
--
INSERT INTO certificate VALUES ('ff80808105416d3b0105416d3eca0001','222');
INSERT INTO certificate VALUES ('ff808081054175b501054175b9190001','222');
--
-- Table structure for table 'student'
--
drop table if exists student;
CREATE TABLE student (
team_id varchar(100) default '',
id varchar(100) NOT NULL default '',
name varchar(20) default '',
`cardId` varchar(20) NOT NULL default '',
age int(11) default '0',
PRIMARY KEY (id)
) ENGINE = InnoDB
CHARACTER SET utf8 COLLATE utf8_general_ci;
--
-- Dumping data for table 'student'
--
INSERT INTO student VALUES ('ff80808105416d3b0105416d3eca0002','ff80808105416d3b0105416d3eca0001','tomclus','200512345',33);
INSERT INTO student VALUES ('ff80808105416d3b0105416d3eca0002','ff808081054175b501054175b9190001','tom','11111111',33);
--
-- Table structure for table 'team'
--
drop table if exists team;
CREATE TABLE team (
id varchar(100) NOT NULL default '',
teamName varchar(100) default '',
PRIMARY KEY (id)
) ENGINE = InnoDB
CHARACTER SET utf8 COLLATE utf8_general_ci;
--
-- Dumping data for table 'team'
--
INSERT INTO team VALUES ('ff80808105416d3b0105416d3eca0002','team1');
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping >
<class name="model.Student" table="student" lazy="true"><!--把类和数表关联起来-->
<id name="id" unsaved-value="null"><!--id的产生方式是uuid.hex-->
<generator class="uuid.hex" />
</id>
<property name="cardId" type="string" /><!--映射号-->
<property name="name" type="string" /><!--映射学生名-->
<property name="age" type="int" /><!--映射学生岁数-->
<many-to-one name="team" column="team_id" class="model.Team" cascade="none" lazy="true"/><!--映射班级-->
</class>
</hibernate-mapping>
Team.hbm.xml
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="model.Team" table="team" lazy="true"><!--把类和数表关联起来-->
<id name="id" unsaved-value="null"><!--id的产生方式是uuid.hex-->
<generator class="uuid.hex" />
</id>
<property name="teamName" type="string" />
<set name="students" lazy="true" cascade="all">
<key column="team_id" />
<one-to-many class="model.Student" />
</set>
<set name="cers" lazy="true" cascade="all">
<key column="team_id" />
<one-to-many class="model.Certificate" />
</set>
</class>
</hibernate-mapping>
二、简单的单表查询
public static void showOneSheet() {
List list = StudentDAO.getAllStu("from Student as s");
for (int i=0;i<list.size();i++) {
Student stu = (Student) list.get(i);
System.out.println(stu.getName());//打印学生名
System.out.println(stu.getTeam().getTeamName());//打印班级名
}
}
则,输出:
Hibernate: select student0_.id as id, student0_.cardId as cardId0_, student0_.name as name0_, student0_.age as age0_, student0_.team_id as team5_0_ from student student0_
Hibernate: select team0_.id as id0_, team0_.teamName as teamName2_0_ from team team0_ where team0_.id=?
两条sql语句。
总结:
1、当查询子类时会立即加载其所对应的父类。
如果在子类的配置文件中配置了多个关联父类,则在查询子类时会将其所对应的多个父类加载出来,即每加载一个父类会连接一次数据库产生一条sql语句。
共产生的sql语句的条数=子类+每个父类
2、访问父类对象时,用子类对象.父类对象即可。
Team t = tu.getTeam();
三、外连接
1、什么时候用外连接?
1)、如果想用一条sql语句查出子类与父类
2)、或是想将符合一个条件的某一张表A中的数据全部输出,不管其连接的另外一张表B中有无表A中的记录。
3)、父类加载子类时,我们一般情况不会在配置文件里设置立即加载。而多数情况是父表对延迟加载。所以这时候用到了 left outer join
2、使用情况:
1)、子表外连接父表
List list = StudentDAO.getAllStu("from Student as s left join s.team ");
Hibernate: select student0_.id as id0_, team1_.id as id1_, student0_.cardId as cardId0_0_, student0_.name as name0_0_, student0_.age as age0_0_, student0_.team_id as team5_0_0_, team1_.teamName as teamName2_1_ from student student0_ left outer join team team1_ on student0_.team_id=team1_.id
个人觉得没必要这样。
2)、父类外连接子类(多数情况)
List list = StudentDAO.getAllStu("from Team t left join t.students ");
Hibernate: select team0_.id as id0_, students1_.id as id1_, team0_.teamName as teamName2_0_, students1_.cardId as cardId0_1_, students1_.name as name0_1_, students1_.age as age0_1_, students1_.team_id as team5_0_1_ from team team0_ left outer join student students1_ on team0_.id=students1_.team_id
配置文件中可以不配置fetch="join" 或outer-join="auto"
取得每个对象:
Object stuAndTeam1[]=(Object[]) list.get(0);//取得第一个数组
Student stu=(Student) stuAndTeam1[0];//
Team t = stu.getTeam();
四、预先抓取
待下篇幅
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