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CREATE TABLE Customers( customer_id int PRIMARY KEY NOT NULL, first_name varchar(50) NULL, last_name varchar(50) NULL, city varchar(50) NULL ) ; CREATE TABLE Sales( sales_id int PRIMARY KEY NOT NULL, sales_date date NULL, customer_id int NULL, product_name varchar(50) NULL, quantity int NULL, price decimal(7, 2) NULL ); ALTER TABLE Sales ADD CONSTRAINT FK_Sales_Customers FOREIGN KEY(customer_id) REFERENCES Customers (customer_id); insert into customers values(1,'alex','rezzani','casteggio'); insert into customers values(2,'john','doe','london'); insert into sales values(1,'2018-09-01',1,'xyz',10,1.5); #--what happens here? insert into sales values(2,'2018-09-01',5,'abc',3,4);