多对多关系——自动删除孤儿

Question

假设球队和球员之间存在多对多关系。这是由下表建模的：

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
);

假设以下数据：

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);

让我们删除团队：

delete from team where identifier = 1;
delete from team where identifier = 2;

现在我们有一个没有团队的球员。有没有办法自动删除这个播放器？也就是说，当删除一个团队导致一个孤儿玩家时，这个玩家也应该被删除（而不是相反）。

Answer 1

嗯，不一定。如果您计划稍后将他添加到团队中，该玩家仍然可以被视为需要团队的玩家。或者，如果您想要拉动可用的玩家。

不过，您可以尝试使用

DELETE FROM member WHERE team_identifier = 1
DELETE FROM member WHERE team_identifier = 2

DELETE FROM player
WHERE Identifier NOT IN (SELECT player_identifier FROM member)

Answer 2

解决您问题的理想方案在成员中创建删除后触发器或更新：

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();

Answer 3

最简单的是添加一个跟踪团队删除的触发器，如下所示：

CREATE TRIGGER drop_orphan_player
  AFTER DELETE ON team FOR EACH ROW
  DELETE FROM player 
  WHERE id NOT IN (
    SELECT DISTINCT player_id FROM member
  );

查看它在 SQLfiddle 的工作情况。

多对多关系——自动删除孤儿

Many-to-many relation - automatically delete orphans

sql

postgresql

many-to-many