Given the following table definition

CREATE TABLE PO$\_$LINE

$($PONR CHAR$(7)$ NOT NULL,

PRODNR CHAR$(6)$ NOT NULL,

QUANTITY INTEGER,

PRIMARY KEY $($PONR$,$ PRODNR$),$

FOREIGN KEY $($PONR$)$ REFERENCES PURCHASE$\_$ORDER $($PONR$)$

ON DELETE RESTRICT ON UPDATE CASCADE,

FOREIGN KEY $($PRODNR$)$ REFERENCES PRODUCT $($PRODNR$)$

ON DELETE SET NULL ON UPDATE SET DEFAULT$)$;

Which statement is NOT CORRECT?