They all do. But these are seperate databases that get translated to text at some point and then converted to Null CREATE TABLE orders ( id INT PRIMARY KE

They all do. But these are seperate databases that get translated to text at some point and then converted to Null CREATE TABLE orders ( id INT PRIMARY KEY, customer_id INT NULL, FOREIGN KEY (customer_id) REFERENCES customers(id) ); vs CREATE TABLE orders ( id INT PRIMARY KEY, customer_id INT NOT NULL, FOREIGN KEY (customer_id) REFERENCES customers(id) );