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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Inheritance</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.24 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Advanced Features" HREF="tutorial-advanced.html"><LINK REL="PREVIOUS" TITLE="Window Functions" HREF="tutorial-window.html"><LINK REL="NEXT" TITLE="Conclusion" HREF="tutorial-conclusion.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2017-11-06T22:43:11"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.24 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Window Functions" HREF="tutorial-window.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="tutorial-advanced.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 3. Advanced Features</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Conclusion" HREF="tutorial-conclusion.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="TUTORIAL-INHERITANCE" >3.6. Inheritance</A ></H1 ><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, altitude int, -- (in ft) state char(2) ); CREATE TABLE non_capitals ( name text, population real, altitude int -- (in ft) ); CREATE VIEW cities AS SELECT name, population, altitude FROM capitals UNION SELECT name, population, altitude 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, altitude int -- (in ft) ); CREATE TABLE capitals ( state char(2) ) INHERITS (cities);</PRE ><P> </P ><P > In this case, a row of <CODE CLASS="CLASSNAME" >capitals</CODE > <I CLASS="FIRSTTERM" >inherits</I > all columns (<TT CLASS="STRUCTFIELD" >name</TT >, <TT CLASS="STRUCTFIELD" >population</TT >, and <TT CLASS="STRUCTFIELD" >altitude</TT >) from its <I CLASS="FIRSTTERM" >parent</I >, <CODE CLASS="CLASSNAME" >cities</CODE >. The type of the column <TT CLASS="STRUCTFIELD" >name</TT > is <TT CLASS="TYPE" >text</TT >, a native <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > type for variable length character strings. State capitals have an extra column, <TT CLASS="STRUCTFIELD" >state</TT >, that shows their state. 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 altitude over 500 feet: </P><PRE CLASS="PROGRAMLISTING" >SELECT name, altitude FROM cities WHERE altitude > 500;</PRE ><P> which returns: </P><PRE CLASS="SCREEN" > name | altitude -----------+---------- 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 altitude of 500 feet or higher: </P><PRE CLASS="PROGRAMLISTING" >SELECT name, altitude FROM ONLY cities WHERE altitude > 500;</PRE ><P> </P><PRE CLASS="SCREEN" > name | altitude -----------+---------- Las Vegas | 2174 Mariposa | 1953 (2 rows)</PRE ><P> </P ><P > Here the <TT CLASS="LITERAL" >ONLY</TT > before <TT CLASS="LITERAL" >cities</TT > 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 — <TT CLASS="COMMAND" >SELECT</TT >, <TT CLASS="COMMAND" >UPDATE</TT >, and <TT CLASS="COMMAND" >DELETE</TT > — support this <TT CLASS="LITERAL" >ONLY</TT > notation. </P ><DIV CLASS="NOTE" ><BLOCKQUOTE CLASS="NOTE" ><P ><B >Note: </B > Although inheritance is frequently useful, it has not been integrated with unique constraints or foreign keys, which limits its usefulness. See <A HREF="ddl-inherit.html" >Section 5.8</A > for more detail. </P ></BLOCKQUOTE ></DIV ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="tutorial-window.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="tutorial-conclusion.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Window Functions</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="tutorial-advanced.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Conclusion</TD ></TR ></TABLE ></DIV ></BODY ></HTML >