[SOLVED] Using foreign keys in SQL

Using foreign keys in SQL

I want to use 'Postcode' from 'address' table which is a primary key there, as a foreign key in 'customers' table. I did the same for getting 'RentalID' from rentals table as a foreign key in 'customers' table, however, when I try doing the same for 'Postcode' it gives me this error: "Please check for a foreign key constraint on table customers for parent table address".

Please note I am using the online SQLite IDE on khanacademy.org

Here is my code:

CREATE TABLE rentals 
( RentalID INTEGER PRIMARY KEY, CarRegistration TEXT, DateHired TEXT, 
DateReturned TEXT); 

INSERT INTO rentals VALUES (  1, "J986NDX", "12.2.94", "25.8.94"); 
INSERT INTO rentals VALUES (  2, "K788NRT", "1.3.95", "1.4.96"); 
INSERT INTO rentals VALUES (  3, "L346NDS", "2.4.96", ""); 
INSERT INTO rentals VALUES (  4, "J986NDX", "15.9.94", "14.9.95"); 
INSERT INTO rentals VALUES (  5, "M411NGT", "15.9.95", "1.2.96");  

CREATE TABLE customers (CustomerID INTEGER, CustomerSurname TEXT, 
Customerinitial TEXT, Companyname TEXT, Postcode TEXT, RentalID INTEGER , 
PRIMARY KEY(CustomerID, RentalID), FOREIGN KEY (RentalID) REFERENCES 
rentals(RentalID), FOREIGN KEY (Postcode) REFERENCES address(Postcode));

INSERT INTO customers VALUES ( 153, "Nazarali", "N", "MF Plastics", "DB5 
3ER", 1); 
INSERT INTO customers VALUES ( 153, "Nazarali", "N", "MF Plastics", "DB5 
3ER", 2); 
INSERT INTO customers VALUES ( 187, "Brown", "L", "MF Plastics", "DB5 3ER", 
3); INSERT INTO customers VALUES ( 287, "Pinner", "M", "Took Ltd", "DB6 
8YU", 4); INSERT INTO customers VALUES ( 287, "Pinner", "M", "Took Ltd", 
"DB6 8YU", 5);  

CREATE TABLE address (Postcode TEXT PRIMARY KEY, town TEXT);  
INSERT INTO address VALUES ( "DB5 3ER", "Derby"); 
INSERT INTO address VALUES ( "DB6 8YU", "Derby");

How can this be solved?

Solution

Basically all I had to do was create the address table and thus create postcode values before I created the customers table because you can't have a foreign key of an attribute that doesn't currently exist, as there would be no constraints. Parent table must be created before customers table.