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<chapter id="advanced">

The inheritance hierarchy is a directed acyclic graph.

</para>

</note>

</para>

<para>

For example, the following query finds the names of all cities,

including state capitals, that are located at an altitude

over 500ft, the query is:

<para>

For example, the following query finds 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

<programlisting>

SELECT c.name, c.altitude

FROM cities c

WHERE c.altitude > 500;

</programlisting>

</programlisting>

which returns:

which returns:

<programlisting>

<programlisting>

+----------+----------+

|name | altitude |

+----------+----------+

+----------+----------+

|Madison | 845 |

+----------+----------+

</programlisting>

</para>

</programlisting>

</para>

<para>

On the other hand, the following query finds

all the cities, but not capital cities

that are situated at an attitude of 500ft or higher:

<para>

On the other hand, the following query finds

all the cities, but not capital cities

that are situated at an attitude of 500ft or higher:

<programlisting>

SELECT name, altitude

<programlisting>

SELECT name, altitude

FROM ONLY cities

WHERE altitude &gt; 500;

+----------+----------+

|Mariposa | 1953 |

+----------+----------+

</programlisting>

</para>

</programlisting>

</para>

<para>

Here the <quote>ONLY</quote> before cities indicates that the query should

be run over only cities and not 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 <quote>ONLY</quote> notation.

</para>

Here the <quote>ONLY</quote> before cities indicates that the query should

be run over only cities and not 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 <quote>ONLY</quote> notation.

</para>

<para>

Deprecated: In previous versions of postgres, the default was not to

get access to child classes. By experience this was found to be error

prone. Under the old syntax, to get the sub-classes you append "*"

to the table name. For example

<programlisting>

SELECT * from cities*;

</programlisting>

This old behaviour is still available by using a SET command...

<programlisting>

SET EXAMINE_SUBCLASS TO on;

</programlisting>

</para>

<para>

Deprecated: In previous versions of postgres, the default was not to

get access to child classes. By experience this was found to be error

prone. Under the old syntax, to get the sub-classes you append "*"

to the table name. For example

<programlisting>

SELECT * from cities*;

</programlisting>

This old behaviour is still available by using a SET command:

<programlisting>

SET EXAMINE_SUBCLASS TO on;

</programlisting>

</para>

</sect1>

<sect1>

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<chapter id="inherit">

The inheritance hierarchy is a actually a directed acyclic graph.

</para>

</note>

</para>

<para>

For example, the following query finds the names of all cities,

</programlisting>

</para>

<para>

Here the <quote>ONLY</quote> before cities indicates that the query should

be run over only cities and not classes below cities in the

inheritance hierarchy. Many of the commands that we