Oracle修改字段长度

DECLARE
CURSOR cur IS
select * from user_tab_columns;
BEGIN
FOR COL IN CUR LOOP
STR VARCHAR2(9999) := '';
STR :=' ALTER TABLE ' ||COL.TABLE_NAME|| ' MODIFY ' ||COL.COLUMN_NAME|| ' VARCHAR2(500) ;';
DBMS_OUTPUT.PUT_LINE(STR);
END LOOP;
END;