MAINHACK

Viewing / Editing File: tutorial-fk.html

File is not writable. Editing disabled.
<!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
>
Cancel / Back
[ MAINHACK ]

Viewing / Editing File: tutorial-fk.html

Server Info

System Features

User Info
