one with a separate <literal>OVER</literal> clause, but this is
    duplicative and error-prone if the same windowing behavior is wanted
    for several functions.  Instead, each windowing behavior can be named
    in a <literal>WINDOW</literal> clause and then referenced in <literal>OVER</literal>.
    For example:

<programlisting>
SELECT sum(salary) OVER w, avg(salary) OVER w
  FROM empsalary
  WINDOW w AS (PARTITION BY depname ORDER BY salary DESC);
</programlisting>
   </para>

   <para>
    More details about window functions can be found in
    <xref linkend="syntax-window-functions"/>,
    <xref linkend="functions-window"/>,
    <xref linkend="queries-window"/>, and the
    <xref linkend="sql-select"/> reference page.
   </para>
  </sect1>


  <sect1 id="tutorial-inheritance">
   <title>Inheritance</title>

   <indexterm zone="tutorial-inheritance">
    <primary>inheritance</primary>
   </indexterm>

   <para>
    Inheritance is a concept from object-oriented databases.  It opens
    up interesting new possibilities of database design.
   </para>

   <para>
    Let's create two tables:  A table <classname>cities</classname>
    and a table <classname>capitals</classname>.  Naturally, capitals
    are also cities, so you want some way to show the capitals
    implicitly when you list all cities.  If you're really clever you
    might invent some scheme like this:

<programlisting>
CREATE TABLE capitals (
  name       text,
  population real,
  elevation  int,    -- (in ft)
  state      char(2)
);

CREATE TABLE non_capitals (
  name       text,
  population real,
  elevation  int     -- (in ft)
);

CREATE VIEW cities AS
  SELECT name, population, elevation FROM capitals
    UNION
  SELECT name, population, elevation FROM non_capitals;
</programlisting>

    This works OK as far as querying goes, but it gets ugly when you
    need to update several rows, for one thing.
   </para>

   <para>
    A better solution is this:

<programlisting>
CREATE TABLE cities (
  name       text,
  population real,
  elevation  int     -- (in ft)
);

CREATE TABLE capitals (
  state      char(2) UNIQUE NOT NULL
) INHERITS (cities);
</programlisting>
   </para>

   <para>
    In this case, a row of <classname>capitals</classname>
    <firstterm>inherits</firstterm> all columns (<structfield>name</structfield>,
    <structfield>population</structfield>, and <structfield>elevation</structfield>) from its
    <firstterm>parent</firstterm>, <classname>cities</classname>.  The
    type of the column <structfield>name</structfield> is