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>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.
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