SQL Add foreign key to existing column

Question

If I am using the following SQL command in SQL Server 2008 to update a table with a foreign key constraint:

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

UserID being my FK column in the Employees table. I'm trying to reference the UserID in my ActiveDirectories table. I receive this error:

Foreign key 'UserID' references invalid column 'UserID' in referencing table 'Employees'.

Answer 1

Error indicates that there is no UserID column in your Employees table. Try adding the column first and then re-run the statement.

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

Answer 2

Maybe you got your columns backwards??

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

Could it be that the column is called ID in the Employees table, and UserID in the ActiveDirectories table?

Then your command should be:

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)

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

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

Answer 4

way of foreign key creation correct for ActiveDirectories(id), i think the main mistake is you didn't mentioned primary key for id in ActiveDirectories table

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

In the future.

ALTER TABLE Employees
ADD UserID int;

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