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