ALTER TABLE ORDERS
   ADD CONSTRAINT FK_ORDER_CUSTOMER FOREIGN KEY (CUSTOMER_ID)
      REFERENCES CUSTOMER (ID);


ALTER TABLE ORDER_ITEM
   ADD CONSTRAINT FK_ORDER_ITEM_ORDER FOREIGN KEY (ORDER_ID)
      REFERENCES ORDERS (ID);


ALTER TABLE ORDER_ITEM
   ADD CONSTRAINT FK_ORDER_ITEM_PRODUCT FOREIGN KEY (PRODUCT_ID)
      REFERENCES PRODUCT (ID);


ALTER TABLE PRODUCT
   ADD CONSTRAINT FK_PRODUCT_SUPPLIER FOREIGN KEY (SUPPLIER_ID)
      REFERENCES SUPPLIER (ID);