SQL将外键添加到现有列

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Employees表格中的FK列。我正在尝试UserID在ActiveDirectories表格中引用。我收到此错误：

外键“ UserID”引用引用表“ Employees”中的无效列“ UserID”。

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

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

也许您的专栏文章倒退了？？

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

难道列被称为ID在Employees表，UserID在ActiveDirectories表？

然后您的命令应该是：

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

MySQL / SQL Server / Oracle / MS Access：

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

若要命名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)

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

ActiveDirectories（id）正确创建外键的方式，我认为主要错误是您没有在ActiveDirectories表中提及ID的主键

在将来。

ALTER TABLE Employees
ADD UserID int;

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