| PostgreSQL 9.1.14 Documentation | ||||
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    Recall the weather and
    cities tables from Chapter 2.  Consider the following problem:  You
    want to make sure that no one can insert rows in the
    weather table that do not have a matching
    entry in the cities table.  This is called
    maintaining the referential integrity of
    your data.  In simplistic database systems this would be
    implemented (if at all) by first looking at the
    cities table to check if a matching record
    exists, and then inserting or rejecting the new
    weather records.  This approach has a
    number of problems and is very inconvenient, so
    PostgreSQL can do this for you.
   
The new declaration of the tables would look like this:
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
);Now try inserting an invalid record:
INSERT INTO weather VALUES ('Berkeley', 45, 53, 0.0, '1994-11-28');
ERROR: insert or update on table "weather" violates foreign key constraint "weather_city_fkey" DETAIL: Key (city)=(Berkeley) is not present in table "cities".
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 Chapter 5 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.