. Consider the following problem: You
want to make sure that no one can insert rows in the
table that do not have a matching
entry in the
table. This is called
your data. In simplistic database systems this would be
implemented (if at all) by first looking at the
table to check if a matching record
exists, and then inserting or rejecting the new
records. This approach has a
number of problems and is very inconvenient, so
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.