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diff --git a/doc/src/sgml/html/tutorial-fk.html b/doc/src/sgml/html/tutorial-fk.html new file mode 100644 index 0000000..2979ee6 --- /dev/null +++ b/doc/src/sgml/html/tutorial-fk.html @@ -0,0 +1,51 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="tutorial-views.html" title="3.2. Views" /><link rel="next" href="tutorial-transactions.html" title="3.4. Transactions" /></head><body id="docContent" class="container-fluid col-10"><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 13.4 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. + </p></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="tutorial-views.html" title="3.2. Views">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="tutorial-advanced.html" title="Chapter 3. Advanced Features">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="tutorial-transactions.html" title="3.4. Transactions">Next</a></td></tr><tr><td width="40%" align="left" valign="top">3.2. Views </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 3.4. Transactions</td></tr></table></div></body></html>
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