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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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