SQL INSERT INTO with empty foreign table - sql

Question

I have a table with 3 values and a foreign table.
Tbl_Person
ID int PRIMARY KEY IDENTITY(1,1)
CarID int REFERENCES Tbl_Car(ID)
Name nvarchar(20)
Tbl_Car
ID int PRIMARY KEY IDENTITY(1,1)
Color nvarchar(30)
I now want to create a new Person with a new Car without caring about the Car table, sort of like this:
INSERT INTO Tbl_Person (Car, Name)
VALUES ('dont know what goes here', 'Timmy')
I'm not sure what to put inside the Car column since I just want it to exist but not care about its values just yet.
I'm sure its quite easy to find online but I have no idea how to Google for this particular problem.

Answer 1

You could alter the logical model to denormalize 'CarID' (remove it from 'Persons' table) and create a new table to store persons' cars called 'Person_Cars'. This avoids inserting NULL values into the model when the car is unknown. Depending on which table constraints are applied it also could allow for persons to have more than 1 car. Also, prefixing tables with 'tbl_' is not necessary. Also, both Persons and Cars seem likely to benefit from a unique constraint on the NVARCHAR column(s).
Persons
ID int not null PRIMARY KEY IDENTITY(1,1)
Name nvarchar(20) unique not null
Cars
ID int not null PRIMARY KEY IDENTITY(1,1)
Car nvarchar(30) unique not null
Person_Cars
ID int not null PRIMARY KEY IDENTITY(1,1)
PersonID int not null REFERENCES Persons(ID)
CarID int not null REFERENCES Cars(ID)

Answer 2

You can use NULL:
CREATE TABLE Tbl_Car (
ID int PRIMARY KEY IDENTITY(1,1),
Color nvarchar(30));
CREATE TABLE Tbl_Person (
ID int PRIMARY KEY IDENTITY(1,1),
CarID int REFERENCES Tbl_Car(ID),
Name nvarchar(20));
INSERT INTO Tbl_Person (CarID, Name) VALUES (NULL, 'Timmy');
DBFIDDLE

Answer 3

You have to do a logical operation. You can not just give the car an ID but it must have a name.
change Tbl_Car to :
CREATE TABLE Tbl_Car (
ID int PRIMARY KEY IDENTITY(1,1),
CarName nvarchar(30),
Color nvarchar(30));
To enter the Tbl_Person table, you must know the name of the person's car :
INSERT INTO Tbl_Person VALUES(CarID,Name)
SELECT ID,'Timmy'
FROM Tbl_Car
WHERE CarName = 'BMW'