diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
new file mode 100644
index c624fc2..ea840fb
*** a/doc/src/sgml/ddl.sgml
--- b/doc/src/sgml/ddl.sgml
*************** CREATE TABLE orders (
*** 688,694 ****
      quantity integer
  );
  </programlisting>
!     Now it is impossible to create orders with
      <structfield>product_no</structfield> entries that do not appear in the
      products table.
     </para>
--- 688,694 ----
      quantity integer
  );
  </programlisting>
!     Now it is impossible to create orders with non-NULL
      <structfield>product_no</structfield> entries that do not appear in the
      products table.
     </para>