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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 (
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 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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