在Postgresql中插入自引用记录 [英] Inserting self-referential records in Postgresql

问题描述

给定PostgreSQL中的下表，我该如何插入一个指向自身的记录？

Given the following table in PostgreSQL, how do I insert a record which refers to itself?

CREATE TABLE refers (
    id        SERIAL  PRIMARY KEY,
    name      VARCHAR(255) NOT NULL,
    parent_id INTEGER      NOT NULL,
    FOREIGN KEY (parent_id) REFERENCES refers(id)
);

我在网上找到的示例允许parent_id为NULL，然后使用触发更新。如果可能的话，我宁愿一口气更新。

The examples I'm finding on the Web have been allowed the parent_id to be NULL and then use a trigger to update it. I'd rather update in one shot, if possible.

推荐答案

您可以从序列中选择last_value，该序列在创建时自动创建您使用串行类型：

You can select last_value from the sequence, that is automatically created when you use type serial:

create table test (
  id serial primary key,
  parent integer not null,
  foreign key (parent) references test(id)
);

insert into test values(default, (select last_value from test_id_seq));
insert into test values(default, (select last_value from test_id_seq));
insert into test values(default, (select last_value from test_id_seq));

select * from test;
 id | parent
----+--------
  1 |      1
  2 |      2
  3 |      3
(3 rows)

以下更简单的方法似乎也有效：

And the following even simpler seems to work as well:

insert into test values(default, lastval());

虽然我不知道使用多个序列时该如何工作...我查了一下; lastval（）返回最后返回的值或使用最后一次对任何序列的nextval或setval调用设置的值，因此以下操作会给您带来麻烦：

Though I don't know how this would work when using multiple sequences... I looked it up; lastval() returns the last value returned or set with the last nextval or setval call to any sequence, so the following would get you in trouble:

create table test (
  id serial primary key,
  foo serial not null,
  parent integer not null,
  foreign key (parent) references test(id)
);

select setval('test_foo_seq', 100);

insert into test values(default, default, lastval());
ERROR:  insert or update on table "test" violates foreign key constraint "test_parent_fkey"
DETAIL:  Key (parent)=(101) is not present in table "test".

不过，可以进行以下操作：

However the following would be okay:

insert into test values(default, default, currval('test_id_seq'));

select * from test;
 id | foo | parent
----+-----+--------
  2 | 102 |      2
(1 row)

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