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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 >Foreign Keys</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="Views" HREF="tutorial-views.html"><LINK REL="NEXT" TITLE="Transactions" HREF="tutorial-transactions.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="Views" HREF="tutorial-views.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="Transactions" HREF="tutorial-transactions.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="TUTORIAL-FK" >3.3. Foreign Keys</A ></H1 ><P > Recall the <CODE CLASS="CLASSNAME" >weather</CODE > and <CODE CLASS="CLASSNAME" >cities</CODE > tables from <A HREF="tutorial-sql.html" >Chapter 2</A >. Consider the following problem: You want to make sure that no one can insert rows in the <CODE CLASS="CLASSNAME" >weather</CODE > table that do not have a matching entry in the <CODE CLASS="CLASSNAME" >cities</CODE > table. This is called maintaining the <I CLASS="FIRSTTERM" >referential integrity</I > of your data. In simplistic database systems this would be implemented (if at all) by first looking at the <CODE CLASS="CLASSNAME" >cities</CODE > table to check if a matching record exists, and then inserting or rejecting the new <CODE CLASS="CLASSNAME" >weather</CODE > records. This approach has a number of problems and is very inconvenient, so <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > can do this for you. </P ><P > The new declaration of the tables would look like this: </P><PRE CLASS="PROGRAMLISTING" >CREATE TABLE cities ( city varchar(80) primary key, location point ); CREATE TABLE weather ( city varchar(80) references cities(city), temp_lo int, temp_hi int, prcp real, date date );</PRE ><P> Now try inserting an invalid record: </P><PRE CLASS="PROGRAMLISTING" >INSERT INTO weather VALUES ('Berkeley', 45, 53, 0.0, '1994-11-28');</PRE ><P> </P><PRE CLASS="SCREEN" >ERROR: insert or update on table "weather" violates foreign key constraint "weather_city_fkey" DETAIL: Key (city)=(Berkeley) is not present in table "cities".</PRE ><P> </P ><P > The behavior of foreign keys can be finely tuned to your application. We will not go beyond this simple example in this tutorial, but just refer you to <A HREF="ddl.html" >Chapter 5</A > for more information. Making correct use of foreign keys will definitely improve the quality of your database applications, so you are strongly encouraged to learn about them. </P ></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-views.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-transactions.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Views</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" >Transactions</TD ></TR ></TABLE ></DIV ></BODY ></HTML >