Question

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  ...

我可以添加什么约束来避免树的循环？

Answer 1

最简单，最常见的树型SQL实现是自引用表，例如：

create table tree(
    id int primary key, 
    parent int references tree(id));

insert into tree values
    (1, null),
    (2, 1),
    (3, 1),
    (4, 2),
    (5, 4);

您可以使用像这样的递归查询从顶部到底部遍历树：

with recursive top_down as (
    select id, parent, array[id] as path
    from tree
    where parent is null
union all
    select t.id, t.parent, path || t.id
    from tree t
    join top_down r on t.parent = r.id
)
select *
from top_down;

 id | parent |   path    
----+--------+-----------
  1 |        | {1}
  2 |      1 | {1,2}
  3 |      1 | {1,3}
  4 |      2 | {1,2,4}
  5 |      4 | {1,2,4,5}
(5 rows)

有关自下而上的示例，另请参见this answer。

完整性

您不能删除作为另一个父节点的节点。外键可防止将树分为不同的部分：

delete from tree
where id = 2;

ERROR:  update or delete on table "tree" violates foreign key constraint "tree_parent_fkey" on table "tree"
DETAIL:  Key (id)=(2) is still referenced from table "tree".

（可选）您可以使用部分唯一索引来确保树只有一个根：

create unique index tree_one_root_idx on tree ((parent is null)) where parent is null;

insert into tree
values(6, null);

ERROR:  duplicate key value violates unique constraint "tree_one_root_idx"
DETAIL:  Key ((parent IS NULL))=(t) already exists.

周期

您可以消除使用触发器输入循环的可能性。该函数检查插入或更新的节点的祖先之一是否可以是节点本身：

create or replace function before_insert_or_update_on_tree()
returns trigger language plpgsql as $$
declare rec record;
begin
    if exists(
        with recursive bottom_up as (
            select new.id, new.parent, array[]::int[] as path, false as cycle
        union all
            select r.id, t.parent, path || t.id, new.id = any(path)
            from tree t
            join bottom_up r on r.parent = t.id and not cycle
        )
        select *
        from bottom_up
        where cycle or (id = parent))
    then raise exception 'Cycle detected on node %.', new.id;
    end if;
    return new;
end $$;

create trigger before_insert_or_update_on_tree
before insert or update on tree
for each row execute procedure before_insert_or_update_on_tree();

检查：

insert into tree values (6, 7), (7, 6);

ERROR:  Cycle detected on node 7.

update tree
set parent = 4
where id = 2;

ERROR:  Cycle detected on node 2.

如何创建没有循环关系的树表？

1 个答案:

完整性

周期