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<!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>3.3. Foreign Keys</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="[email protected]" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="tutorial-views.html" title="3.2. Views" /><link rel="next" href="tutorial-transactions.html" title="3.4. Transactions" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">3.3. Foreign Keys</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tutorial-views.html" title="3.2. Views">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 10.23 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="tutorial-transactions.html" title="3.4. Transactions">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="TUTORIAL-FK"><div class="titlepage"><div><div><h2 class="title" style="clear: both">3.3. Foreign Keys</h2></div></div></div><a id="id-1.4.5.4.2" class="indexterm"></a><a id="id-1.4.5.4.3" class="indexterm"></a><p> Recall the <code class="classname">weather</code> and
<code class="classname">cities</code> tables from <a class="xref" href="tutorial-sql.html" title="Chapter 2. The SQL Language">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 <em class="firstterm">referential integrity</em> 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 (
name varchar(80) primary key,
location point
);
CREATE TABLE weather (
city varchar(80) references cities(name),
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 class="xref" href="ddl.html" title="Chapter 5. Data Definition">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.
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