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diff --git a/doc/src/sgml/html/tutorial-inheritance.html b/doc/src/sgml/html/tutorial-inheritance.html new file mode 100644 index 0000000..a254e97 --- /dev/null +++ b/doc/src/sgml/html/tutorial-inheritance.html @@ -0,0 +1,113 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>3.6. Inheritance</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="tutorial-window.html" title="3.5. Window Functions" /><link rel="next" href="tutorial-conclusion.html" title="3.7. Conclusion" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">3.6. Inheritance</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tutorial-window.html" title="3.5. Window Functions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="tutorial-advanced.html" title="Chapter 3. Advanced Features">Up</a></td><th width="60%" align="center">Chapter 3. Advanced Features</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="tutorial-conclusion.html" title="3.7. Conclusion">Next</a></td></tr></table><hr /></div><div class="sect1" id="TUTORIAL-INHERITANCE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">3.6. Inheritance <a href="#TUTORIAL-INHERITANCE" class="id_link">#</a></h2></div></div></div><a id="id-1.4.5.7.2" class="indexterm"></a><p> + Inheritance is a concept from object-oriented databases. It opens + up interesting new possibilities of database design. + </p><p> + Let's create two tables: A table <code class="classname">cities</code> + and a table <code class="classname">capitals</code>. 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: + +</p><pre class="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; +</pre><p> + + This works OK as far as querying goes, but it gets ugly when you + need to update several rows, for one thing. + </p><p> + A better solution is this: + +</p><pre class="programlisting"> +CREATE TABLE cities ( + name text, + population real, + elevation int -- (in ft) +); + +CREATE TABLE capitals ( + state char(2) UNIQUE NOT NULL +) INHERITS (cities); +</pre><p> + </p><p> + In this case, a row of <code class="classname">capitals</code> + <em class="firstterm">inherits</em> all columns (<code class="structfield">name</code>, + <code class="structfield">population</code>, and <code class="structfield">elevation</code>) from its + <em class="firstterm">parent</em>, <code class="classname">cities</code>. The + type of the column <code class="structfield">name</code> is + <code class="type">text</code>, a native <span class="productname">PostgreSQL</span> + type for variable length character strings. The + <code class="classname">capitals</code> table has + an additional column, <code class="structfield">state</code>, which shows its + state abbreviation. In + <span class="productname">PostgreSQL</span>, a table can inherit from + zero or more other tables. + </p><p> + For example, the following query finds the names of all cities, + including state capitals, that are located at an elevation + over 500 feet: + +</p><pre class="programlisting"> +SELECT name, elevation + FROM cities + WHERE elevation > 500; +</pre><p> + + which returns: + +</p><pre class="screen"> + name | elevation +-----------+----------- + Las Vegas | 2174 + Mariposa | 1953 + Madison | 845 +(3 rows) +</pre><p> + </p><p> + On the other hand, the following query finds + all the cities that are not state capitals and + are situated at an elevation over 500 feet: + +</p><pre class="programlisting"> +SELECT name, elevation + FROM ONLY cities + WHERE elevation > 500; +</pre><p> + +</p><pre class="screen"> + name | elevation +-----------+----------- + Las Vegas | 2174 + Mariposa | 1953 +(2 rows) +</pre><p> + </p><p> + Here the <code class="literal">ONLY</code> before <code class="literal">cities</code> + indicates that the query should be run over only the + <code class="classname">cities</code> table, and not tables below + <code class="classname">cities</code> in the inheritance hierarchy. Many + of the commands that we have already discussed — + <code class="command">SELECT</code>, <code class="command">UPDATE</code>, and + <code class="command">DELETE</code> — support this <code class="literal">ONLY</code> + notation. + </p><div class="note"><h3 class="title">Note</h3><p> + Although inheritance is frequently useful, it has not been integrated + with unique constraints or foreign keys, which limits its usefulness. + See <a class="xref" href="ddl-inherit.html" title="5.10. Inheritance">Section 5.10</a> for more detail. + </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="tutorial-window.html" title="3.5. Window Functions">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="tutorial-advanced.html" title="Chapter 3. Advanced Features">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="tutorial-conclusion.html" title="3.7. Conclusion">Next</a></td></tr><tr><td width="40%" align="left" valign="top">3.5. Window Functions </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 3.7. Conclusion</td></tr></table></div></body></html>
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