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)
);
