sql-server-2008 - SQL将外键添加到现有列

Question

如果我在 SQL Server 2008 中使用以下 SQL 命令来更新具有外键约束的表：

ALTER TABLE Employees
ADD FOREIGN KEY (UserID)
REFERENCES ActiveDirectories(id)

UserID是我在表中的 FK 列Employees。我试图UserID在我的ActiveDirectories表中引用。我收到此错误：

外键“UserID”在引用表“Employees”中引用了无效列“UserID”。

Answer 1

错误表明您的员工表中没有 UserID 列。尝试先添加列，然后重新运行语句。

ALTER TABLE Employees
ADD CONSTRAINT FK_ActiveDirectories_UserID FOREIGN KEY (UserID)
    REFERENCES ActiveDirectories(id);

Answer 2

也许你的列倒了？

ALTER TABLE Employees
ADD FOREIGN KEY (UserID)           <-- this needs to be a column of the Employees table
REFERENCES ActiveDirectories(id)   <-- this needs to be a column of the ActiveDirectories table

难道是在表中调用ID了该Employees列，并且UserID在ActiveDirectories表中？

那么你的命令应该是：

ALTER TABLE Employees
ADD FOREIGN KEY (ID)                   <-- column in table "Employees"
REFERENCES ActiveDirectories(UserID)   <-- column in table "ActiveDirectories" 

Answer 3

MySQL / SQL Server / Oracle / MS Access：

ALTER TABLE Orders
ADD FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)

要允许命名 FOREIGN KEY 约束并在多个列上定义 FOREIGN KEY 约束，请使用以下 SQL 语法：

MySQL / SQL Server / Oracle / MS Access：

ALTER TABLE Orders
ADD CONSTRAINT fk_PerOrders
FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)

Answer 4

在将来。

ALTER TABLE Employees
ADD UserID int;

ALTER TABLE Employees
ADD CONSTRAINT FK_ActiveDirectories_UserID FOREIGN KEY (UserID)
    REFERENCES ActiveDirectories(id);

Answer 5

ALTER TABLE Faculty 
WITH CHECK ADD  CONSTRAINT FKFacultyBook
FOREIGN KEY FacId
REFERENCES Book Book_Id

ALTER TABLE Faculty 
WITH CHECK ADD  CONSTRAINT FKFacultyStudent 
FOREIGN KEY FacId
REFERENCES Student StuId

Answer 6

ActiveDirectories（id）的外键创建方式正确，我认为主要错误是您没有在ActiveDirectories表中提到id的主键

Answer 7

如果表已经创建：

首先做：

ALTER TABLE `table1_name` ADD UNIQUE( `column_name`);

然后：

ALTER TABLE `table1_name` ADD FOREIGN KEY (`column_name`) REFERENCES `table2_name`(`column_name`);