您好，



首先，您可以只有一个主键。您的主键中可以有多个列。这样的密钥称为复合或复合主键。



根据您的示例：

 创建  table 项目（
 Item_UID  int ， 
 Item_ID  varchar （ 15 ），
 Item_Description  varchar （ 300 ）
 primary   key （Item_UID），
约束 fk_PerItem  foreign   key （Item_ID） references  ItemsInOrder（ Order_Num，Item_ID ））; 

这很重要：您需要在项表中添加 Order_Num 列（我尚未将其添加到此示例中） 。否则，你就是无法建立外键关系！



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我对你的数据模型的看法：

1.为什么你需要一个复合主键 ItemsInOrder 表？我会说你可以添加 ItemsInOrder_ID 列并使其成为简单的主键。

2. 项目与 ItemsInOrder 表格应该是一对多。

3. 订单和 ItemsInOrder 表之间的关系应该是一对多。

use LifeSpireDB;


create table Customer(
Customer_ID int,
Customer_Name varchar(50),
Customer_Address varchar(50),
Customer_Phone varchar(50),
primary key (customer_ID));

create table Orders(
Order_Num int,
Order_Date varchar(10),
Order_Status varchar(50),
Customer_ID int,
primary key(Order_Num),
constraint fk_PerCostumer foreign key(Customer_ID) references Customer(Customer_ID));

create table ItemsInOrder(
Order_Num int,
Item_ID varchar(15)
primary key (Order_Num, Item_ID),
constraint fk_perItemsOder foreign key(Order_Num) references Orders(Order_Num));

create table Item(
Item_UID int,
Item_ID varchar(15),
Item_Description varchar(300)
primary key (Item_UID),
constraint fk_PerItem foreign key (Item_ID) references ItemsInOrder(Item_ID));



My table Item has a foreign key which is a primary key in table ItemsInOrder, the problem is the ItemsInOrder has two primary keys.

Can anyone help me please???

Hi,

First of all, you can have only one primary key. You can have multiple columns in your primary key. Such key is called a compound or composite primary key.

According to your example:

create table Item(
Item_UID int,
Item_ID varchar(15),
Item_Description varchar(300)
primary key (Item_UID),
constraint fk_PerItem foreign key (Item_ID) references ItemsInOrder(Order_Num, Item_ID));

This is important: You need to have a Order_Num column in your Item table (I haven't added it into this example). Otherwise, you just can't create a foreign key relationship!

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My thoughts about your data model:
1. Why do you need a composite primary key in ItemsInOrder table? I would say that you could add ItemsInOrder_ID column and make it as simple primary key.
2. Relationship between Item and ItemsInOrder tables should be one-to-many.
3. Relationship between Orders and ItemsInOrder tables should be one-to-many.