Re: Deletion Recursively

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What you need are foreign keys with a cascade delete. Here
is a quick example:

CREATE TABLE continent (
continent_id INTEGER NOT NULL PRIMARY KEY,
continent_name VARCHAR (20)
);

CREATE TABLE country (
country_id INTEGER NOT NULL PRIMARY KEY,
country_name VARCHAR(40),
continent_id INTEGER,
CONSTRAINT has_continent
FOREIGN KEY(continent_id) REFERENCES continent
ON DELETE CASCADE
);

CREATE TABLE city (
city_id INTEGER NOT NULL PRIMARY KEY,
city_name VARCHAR(100),
country_id INTEGER,
CONSTRAINT has_country
FOREIGN KEY(country_id) REFERENCES country
ON DELETE CASCADE
);

Now we add some continents:

INSERT INTO continent(continent_id, continent_name)
VALUES (1, 'Africa');
INSERT INTO continent(continent_id, continent_name)
VALUES (2, 'Europe');
INSERT INTO continent(continent_id, continent_name)
VALUES (3, 'Atlantis');

Then some countries:

INSERT INTO country(country_id, country_name, continent_id)
VALUES (1,'France',2);
INSERT INTO country(country_id, country_name, continent_id)
VALUES (2,'Xanadu',3);
INSERT INTO country(country_id, country_name, continent_id)
VALUES (3,'Spain',2);
INSERT INTO country(country_id, country_name, continent_id)
VALUES (4,'Kenya',1);
INSERT INTO country(country_id, country_name, continent_id)
VALUES (5,'Ethiopia',1);
INSERT INTO country(country_id, country_name, continent_id)
VALUES (6,'Fantasia',3);

And finally a few cities:

INSERT INTO city(city_id, city_name, country_id)
VALUES(1, 'Paris', 1);
INSERT INTO city(city_id, city_name, country_id)
VALUES(2, 'Lisbon', 3);
INSERT INTO city(city_id, city_name, country_id)
VALUES(3, 'Aquala', 2);
INSERT INTO city(city_id, city_name, country_id)
VALUES(4, 'Mombasa', 4);
INSERT INTO city(city_id, city_name, country_id)
VALUES(5, 'Aragornia', 6);
INSERT INTO city(city_id, city_name, country_id)
VALUES(6, 'Brie', 1);

Let's take a look at what we have. We'll list all the
cities that are in the 'city' table, and also show
which country and continent they belong to:

SELECT city_name, country_name, continent_name
FROM city C1, country C2, continent C3
WHERE C1.country_id=C2.country_id
AND C2.continent_id = C3.continent_id;

city_name | country_name | continent_name
- -----------+--------------+----------------
Mombasa | Kenya | Africa
Paris | France | Europe
Brie | France | Europe
Lisbon | Spain | Europe
Aquala | Xanadu | Atlantis
Aragornia | Fantasia | Atlantis
(6 rows)

As you can see, there are 6 rows, for 6 cities. Let's get rid
of Atlantis, since it does not really exists. When we do this,
it also gets rid of any countries within Atlantis, as well as
any cities within those countries. The deletes cascade down from
the original level, down to the countries, and then down to the
cities:

DELETE FROM continent WHERE continent_name='Atlantis';

Now let's take a look using the same SELECT statement:

city_name | country_name | continent_name
- -----------+--------------+----------------
Mombasa | Kenya | Africa
Paris | France | Europe
Brie | France | Europe
Lisbon | Spain | Europe
(4 rows)

Only 4 rows! Not only did we delete rows from the continent
table, but also from the country and the city tables as well.

This was a crude example: for example, the primary keys used
should really be sequences, so that we don't have to worry
about keeping track of them ourselves. But hopefully it
illustrates foreign keys and a cascade delete.

Greg Sabino Mullane
greg(at)turnstep(dot)com
PGP Key: 0x14964AC8 200110291549

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