Thread: Deletion Recursively

On Fri, 26 Oct 2001, Michael Dyrby Jensen wrote:

> Greetings.
>
> I am fairly new with the postgresql database.
> And I have run into a problem, which I hope someone can help me with.
>
> I am doing a school project, where I have chosen to learn to use this powerful tool, so I am using it to
> handle a fictive company of banner advertising.
>
> I need to register Advertisers, who got some Campaigns, and these
> campaigns have banners.
> I have an Advertiser table, a campaign table and a banner table.
>
> an advertiser hold references to the campaigns that he own, and the
> campaigns have references to the banners that they own.
>
> My question is then: "How do I make the Database delete the campaigns
> and banners linked to the advertiser, when I delete him?"
>
> A small example would really really help me alot.
> (with small explanation too ofcause =) )

If I'm understanding your problem correctly...
(untested, but I think I got the syntax stuff right)

create table advertiser (advid serial primary key,name varchar,...
);

create table campaigns (campid serial primary key,advid integer references advertisers(advid) on delete cascade on
updatecascade,-- Make a foreign key constraint to the advertiser-- table such that a campaign must either have a-- NULL
advidor a valid one at all times.  If the-- advid is updated in advertiser, all of the ones-- in campaign that
referencesthat one are updated-- (on update cascade).  If a row is deleted in-- advertiser, all rows in campaign that
reference--that one are deleted (on delete cascade)...
 
);

create table banners (bannerid serial,campid integer references campaigns(campid) on delete cascade on update
cascade,...
);

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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,
CONSTRAINThas_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        | AfricaParis     | France       | EuropeBrie
| France       | EuropeLisbon    | Spain        | EuropeAquala    | Xanadu       | AtlantisAragornia | 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        | AfricaParis     | France       | EuropeBrie
| France       | EuropeLisbon    | 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@turnstep.com
PGP Key: 0x14964AC8 200110291549

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