There are no Primary or Candidate Keys in the referenced table

Question

Error: There are no Primary or Candidate Keys in the referenced table 'dbo.Customers' that match the referencing column list in the foreign key 'FK_Reservation_Customers_FrstNme FOREIGN KEY'

DROP TABLE dbo.Customers;
DROP TABLE dbo.Staff;
DROP TABLE dbo.Rooms;
DROP TABLE dbo.Reservation;
GO
CREATE TABLE "Customers"(

    CustomerID int IDENTITY (1,1) NOT NULL,
    FirstName nvarchar(20) NULL,
    LastName nvarchar(20) NULL,
    StreetNo int NULL,
    City nvarchar(20) NULL,
    PostCode nvarchar(20) NULL,
    Email nvarchar(50) NULL,

    CONSTRAINT PK_Customers PRIMARY KEY
    (
        CustomerID
    )
)
CREATE TABLE "Staff"(

    StaffID nvarchar(20) NOT NULL,
    Pass nvarchar(20) NOT NULL,

    CONSTRAINT PK_Staff PRIMARY KEY
    (
        StaffID
    )
)
CREATE TABLE "Rooms"(

    RoomNo int NOT NULL,
    RoomType nvarchar(20) NULL,
    PricePerNight money NULL,
    MaximumOccupancy int NULL,
    No0fBeds int NULL,
    NoOfBathrooms int NULL,
    Entertainment bit NULL,
    RoomService bit NULL,
    Gym bit NULL,

    CONSTRAINT PK_Rooms PRIMARY KEY
    (
        RoomNo
    )
)
CREATE TABLE "Reservation"(

    ReservationID int IDENTITY (1,1) NOT NULL,
    CustomerID int NOT NULL,
    FirstName nvarchar(20) NULL,
    LastName nvarchar(20) NULL,
    RoomType nvarchar(20) NULL,
    RoomNo int NOT NULL,
    CheckInDate date NULL,
    CheckOutDate date NULL,

    CONSTRAINT PK_Reservation PRIMARY KEY
    (
        ReservationID
    ),
    CONSTRAINT FK_Reservation_Customers_CustID FOREIGN KEY
    (
        CustomerID
    )   
        REFERENCES dbo.Customers
        (
            CustomerID
        ),
    CONSTRAINT FK_Reservation_Customers_FrstNme FOREIGN KEY
    (
        FirstName
    )
        REFERENCES dbo.Customers
        (
            FirstName
        )
    )

Could someone please tell me whats happening here and how i can fix it. Same problem occurs with all the other keys i want to make a foreign key. Except if i want to reference a primary key.

Answer 1

If you want to create a foreign key, it must reference either the primary key, or a field with a unique constraint.

If you want to display the customer's name, make the foreign key reference the CustomerID, and display the results with a join.

Answer 2

I ran into this error when specifying key columns in the referenced table that were out of the same order that the primary key or candidate key was defined with on the reference table.

Switching the order the columns were specified fixed it.

Ex:

ALTER TABLE [dbo].[MCL_item_vendor_override_mst]  WITH CHECK ADD  
CONSTRAINT [FK_MCL_item_vendor_override_mst_item_mst] FOREIGN KEY([item],[site_ref])
    REFERENCES [dbo].[item_mst] ([item],[site_ref])

failed while

ALTER TABLE [dbo].[MCL_item_vendor_override_mst]  WITH CHECK ADD  
CONSTRAINT [FK_MCL_item_vendor_override_mst_item_mst] FOREIGN KEY([site_ref],[item])
    REFERENCES [dbo].[item_mst] ([site_ref],[item])

worked.