JDBC资料辅导（4）

一、ID的High/Low算法 　　高位数字分别与低位数字相匹配，得到的数字是唯一的 　　减少与数据库的交互

二、ORM １、类映射成表 　　类名与表名对应 ２、属性定义映射成列，类型之间必须是兼容的 ３、类关系映射成表关系

一对一双向关系 内存中都保存对方的一个引用 数据库中，表b的id是主键，也是外键，引用a表的id主键　－－　share pk           表b中有一个字段aid是外键，引用a表的主键，并且有唯一约束　　－－　pk+fk 共享主键： create table car_pk (     id number(10,0) not null,     name varchar2(15),     serial varchar2(30),     manufacturer varchar2(50),     producedate date,     primary key (id) );

create table engine_pk (     id number(10,0) not null,     model varchar2(20),     manufacturer varchar2(50),     producedate date,     primary key (id) );

alter table engine_pk     add constraint fk_engine_car_pk     foreign key (id)     references car_pk(id); 外键＋唯一约束 create table car_fk (     id number(10,0) not null,     name varchar2(15) not null,     serial varchar2(30) not null,     manufacturer varchar2(50) not null,     producedate date,     primary key (id) );

create table engine_fk (     id number(10,0) not null,     model varchar2(20) not null,     manufacturer varchar2(50) not null,     producedate date,     carid number(10,0) unique,     primary key (id) );

alter table engine_fk     add constraint fk_engine_car_fk     foreign key (carid)     references car_fk(id);              实体对象：在内存中有id属性的 值对象：没有id的，依赖其他对象存在

一对多关系          一的一方保存多一方的一个集合，最好使用set，保证无重复元素 多的一方保存一一方的一个对象的引用 public class Order implements Serializable{     private int id;     private String owner;     private String phone;     private String address;     private Set<Item> items = new HashSet<Item>(); } public class Item implements Serializable{     private int id;     private String product;     private int amount;     private Order order;    }   

create table ec_item (     id number(10,0) not null,     product varchar2(15) not null,     amount number(10,0) not null,     orderid number(10,0) not null,     primary key (id) );

create table ec_order (     id number(10,0) not null,     owner varchar2(15) not null,     phone varchar2(15) not null,     address varchar2(50),     primary key (id) );

alter table ec_item     add constraint fk_item_order     foreign key (orderid)     references ec_order(id); 多对多 双方都保存对方的多个引用 例子：学生选课 public class TarenaCourse implements Serializable{     private int id;     private String name;     private int period;     private Set<TarenaStudent> students = new HashSet<TarenaStudent>();    } public class TarenaStudent implements Serializable{     private int id;     private String name;     private Date birthday;     private Set<TarenaCourse> courses = new HashSet<TarenaCourse>();    }

create table student (     id number(10,0) not null,     name varchar2(15) not null,     birthday date,     primary key (id) );

create table student_course (     sid number(10,0) not null,     cid number(10,0) not null,     primary key (sid, cid) );

create table course (     id number(10,0) not null,     name varchar2(15) not null,     perion number(10,0),     primary key (id) );

alter table student_course     add constraint fk_student     foreign key (sid)     references student(id);

alter table student_course     add constraint fk_course     foreign key (cid)     references course(id);

通过学生姓名找课程    select c.name from cource c,student s,student_course sc where c.id=sc.cid and  s.id=sc.sid and s.name = 's1' 

三、继承关系     public abstract class Computer implements Serializable{     private int id;     private int price;     private String manufacturer; }

public class Desktop extends Computer{     private boolean isLCD;    }

public class Notepad extends Computer{     private float weight;     private float thickness;    } 

１、建３张表　table per class 子类中保存父类的主键作为外键 create table computer_tpc (     id number(10,0) not null,     price number(10,0) not null,     manufacturer varchar2(30) not null,     primary key (id) );

create table desktop_tpc (     computerid number(10,0) not null,     islcd char(1),     primary key (computerid) );

create table notepad_tpc (     computerid number(10,0) not null,     weight float,     thickness float,     primary key (computerid) );

alter table desktop_tpc     add constraint fk_desk_computer_tpc     foreign key (computerid)     references computer_tpc(id);

alter table notepad_tpc     add constraint fk_note_computer_tpc     foreign key (computerid)     references computer_tpc(id);

查找所有电脑的配制（只要是电脑就能被查出来）    select c.id,c.price,d.islcd,n.weight,n.thickness from computer c, desktop d,notepad n where  c.id = d.computerid(+) and  c.id = n.computer(+) ２、建２张表      create table desktop (     id number(10,0) not null,     price number(10,0) not null,     manufacturer varchar2(30) not null,     islcd char(1),     primary key (id) );

create table notepad (     id number(10,0) not null,     price number(10,0) not null,     manufacturer varchar2(30) not null,     weight float,     thickness float,     primary key (id) );

３、建１张表 create table computer_tph (     id number(10,0) not null,     category char(1) not null,     price number(10,0) not null,     manufacturer varchar2(30) not null,     islcd char(1),     weight float,     thickness float,     primary key (id) );