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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/Attic/inherit.sgml,v 1.7 2000/05/02 20:01:51 thomas Exp $
-->
<chapter id="inherit">
<title>Inheritance</title>
<para>
Let's create two classes. The capitals class contains
state capitals which are also cities. Naturally, the
capitals class should inherit from cities.
<programlisting>
CREATE TABLE cities (
name text,
population float,
altitude int -- (in ft)
);
CREATE TABLE capitals (
state char(2)
) INHERITS (cities);
</programlisting>
In this case, an instance of capitals <firstterm>inherits</firstterm> all
attributes (name, population, and altitude) from its
parent, cities. The type of the attribute name is
<type>text</type>, a native <productname>Postgres</productname> type for variable length
ASCII strings. The type of the attribute population is
<type>float</type>, a native <productname>Postgres</productname> type for double precision
floating point numbers. State capitals have an extra
attribute, state, that shows their state. In <productname>Postgres</productname>,
a class can inherit from zero or more other classes,
and a query can reference either all instances of a
class or all instances of a class plus all of its
descendants.
<note>
<para>
The inheritance hierarchy is a actually a directed acyclic graph.
</para>
</note>
For example, the following query finds
all the cities that are situated at an attitude of 500ft or higher:
<programlisting>
SELECT name, altitude
FROM cities
WHERE altitude > 500;
name | altitude
-----------+----------
Las Vegas | 2174
Mariposa | 1953
(2 rows)
</programlisting>
</para>
<para>
On the other hand, to find the names of all cities,
including state capitals, that are located at an altitude
over 500ft, the query is:
<programlisting>
SELECT c.name, c.altitude
FROM cities* c
WHERE c.altitude > 500;
</programlisting>
which returns:
<programlisting>
name | altitude
-----------+----------
Las Vegas | 2174
Mariposa | 1953
Madison | 845
</programlisting>
Here the "*" after cities indicates that the query should
be run over cities and all classes below cities in the
inheritance hierarchy. Many of the commands that we
have already discussed -- <command>SELECT</command>,
<command>UPDATE</command> and <command>DELETE</command> --
support this "*" notation, as do others, like
<command>ALTER TABLE</command>.
</para>
</chapter>
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End:
-->
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