Assume a many-to-many relation between team and player. This is modelled by the following tables:
create table team
(
identifier integer primary key
);
create table player
(
identifier integer primary key
);
create table member
(
team_identifier integer,
player_identifier integer,
primary key(team_identifier, player_identifier),
foreign key(team_identifier) references team on update cascade on delete cascade,
foreign key(player_identifier) references player on update cascade on delete cascade
);
Assume the following data:
insert into team values(1);
insert into team values(2);
insert into player values(1);
insert into member values(1, 1);
insert into member values(2, 1);
Let's delete the teams:
delete from team where identifier = 1;
delete from team where identifier = 2;
Now we have a player without a team. Is there a way to automatically delete this player? That is, when the deletion of a team results in an orphan player, this player should be removed as well (but not the other way around).
ideal solution to settle your problem create after-delete trigger or update in membre:
CREATE FUNCTION delete_player_not_in_member() RETURNS trigger AS $delete_player_not_in_member$
BEGIN
DELETE FROM player WHERE Identifier NOT IN (SELECT player_identifier FROM membre);
RETURN OLD;
END;
$delete_player_not_in_member$ LANGUAGE plpgsql;
CREATE TRIGGER delete_player_not_in_member AFTER DELETE OR UPDATE ON member
FOR EACH ROW EXECUTE PROCEDURE delete_player_not_in_member();