Question

/*Create Purchased Products Table*/
create table Purchased_Products
(
School_ID INT not null,
Product_ID INT not null,
Purchased_Product_ID INT IDENTITY (1,1) not null,
Quantity INT,
Product_Status VARCHAR(1)

  constraint Purchased_Products_PK PRIMARY KEY (School_ID, Product_ID, Purchased_Product_ID),
  constraint Purchased_Products_FK1 FOREIGN KEY (School_ID) REFERENCES School(School_ID),
  constraint Purchased_Products_FK2 FOREIGN KEY (Product_ID) REFERENCES Product(Product_ID)
);

create table Credentials
(
Credential_ID INT IDENTITY (1,1) not null,
Purchased_Product_ID INT not null,
Username VARCHAR (30),
PPassword VARCHAR (30),
URL VARCHAR (100),
Remote_Username VARCHAR (30),
Remote_Password VARCHAR (30),
Remote_URL VARCHAR (100),
Notes VARCHAR (200),

  constraint Credentials_PK PRIMARY KEY (Credential_ID, Purchased_Product_ID),
  constraint Credentials_FK FOREIGN KEY (Purchased_Product_ID) REFERENCES Purchased_Products(Purchased_Product_ID)

);

好的，所以我想创建这个表，Credentials，但它给了我错误：

Msg 1776，Level 16，State 0，Line 1 引用的表'Purchased_Products'中没有与外键'Credentials_FK'中的引用列列表匹配的主键或候选键。 Msg 1750，Level 16，State 0，Line 1 无法创建约束。查看以前的错误。

它似乎允许我这样做，但我想不出我在做什么。也许我忽略了什么？帮助?? !! ??

Answer 1

Purchased_Products.Purchased_Product_ID需要显式为UNIQUE或显式主键，才能使其成为外键的参考列。

CREATE UNIQUE NONCLUSTERED INDEX IndexName 
ON Purchased_Products(Purchased_Product_ID)

如果索引中有多个列，则创建唯一索引会对列（或列集）强制执行唯一性，SQL将允许您将其用作外键的引用。

Answer 2

外键只能在具有UNIQUE coumns的列上创建，例如主键或唯一索引。

表Purchased_Products目前只有一个唯一约束，即复合PK：

constraint Purchased_Products_PK 
   PRIMARY KEY (School_ID, Product_ID, Purchased_Product_ID)

表Credentials正试图仅在其中一列上强制执行外键，即

constraint Credentials_FK 
  FOREIGN KEY (Purchased_Product_ID) 
  REFERENCES Purchased_Products(Purchased_Product_ID)

从技术上讲，您需要重复Credentials中的所有复合键列（即School_ID，Product_ID，Purchased_Product_ID）并在这些列上创建复合外键。

但是，您的表格设计似乎有问题 - 您选择Primary Keys似乎很不幸，例如Purchased_Products有一个简单代理主键的候选者：

Purchased_Product_ID INT IDENTITY (1,1) not null

同样对于Credentials，鉴于Credential_ID已identity已唯一，因此无需向PK添加Purchased_Product_ID。你可能想要的是：

Table : Products
Primary Key : ProductId

Table : Purchased_Products
Primary Key : Purchased_Product_ID
Foreign Key : ProductID references Products(ProductId)

Table : Credentials
Primary Key : Credential_ID
Foreign Key : Purchased_Product_ID references Purchased_Products(Purchased_Product_ID)

等

为什么我的外键约束不起作用？

2 个答案: