Can a value be a primary and a foreign key? - sql

Question

I have the following tables in my SQL Server database.
Customer is a subtype of User thus I made a reference from Customer to User.
I also want to link my customer to my reservation. I'd like to use the Foreign key in the Customer table as a PK for this relation. When I write this out in SQL Server, I get this error under "(userid)":
Invalid column 'userid'
How can I make this relation?
Create table [dbo].[User]
(
[id] int PRIMARY KEY IDENTITY (1,1) NOT NULL,
[name] varchar(50) NOT NULL,
[password] nvarchar(100) NOT NULL,
)
Create table [dbo].[Customer]
(
[userid] int FOREIGN KEY REFERENCES [dbo].[User](id) NOT NULL,
[street] varchar(40) NOT NULL,
[housenumber] varchar(10) NOT NULL,
[postalcode] varchar(10) NOT NULL,
[phonenumber] varchar(20) NOT NULL,
[email] varchar(30) NOT NULL,
)
Create table [dbo].[Reservation]
(
[id] int PRIMARY KEY IDENTITY (1,1) NOT NULL,
[datum] date NOT NULL,
[prijs] decimal NOT NULL,
[levertijd] datetime NOT NULL,
[ophaaltijd] datetime NOT NULL,
[leverAdres] varchar(60) NOT NULL,
[klantId] int FOREIGN KEY REFERENCES [dbo].[Customer](userid) NOT NULL
)

Answer 1

yes this is possible, try it like this
Create table [dbo].[Customer]
(
[userid] int not null,
[street] varchar(40) NOT NULL,
[housenumber] varchar(10) NOT NULL,
[postalcode] varchar(10) NOT NULL,
[phonenumber] varchar(20) NOT NULL,
[email] varchar(30) NOT NULL,
constraint PK_UserID primary key ([userid]),
constraint FK_Customer_User foreign key (userid) references [User] (id)
)
But I have to say that userid seems like an odd primary key for a table called Customer
I would so something like this :
Create table [dbo].[Customer]
(
[CustomerID] int not null identity,
[userid] int not null,
[street] varchar(40) NOT NULL,
[housenumber] varchar(10) NOT NULL,
[postalcode] varchar(10) NOT NULL,
[phonenumber] varchar(20) NOT NULL,
[email] varchar(30) NOT NULL,
constraint PK_CustomerID primary key ([CustomerID]),
constraint FK_Customer_User foreign key (userid) references [User] (id)
)
Create table [dbo].[Reservation]
(
[id] int PRIMARY KEY IDENTITY (1,1) NOT NULL,
[datum] date NOT NULL,
[prijs] decimal NOT NULL,
[levertijd] datetime NOT NULL,
[ophaaltijd] datetime NOT NULL,
[leverAdres] varchar(60) NOT NULL,
[klantId] int FOREIGN KEY REFERENCES [dbo].[Customer](customerid) NOT NULL
)