Inserting self-referential records in Postgresql
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)
);
The examples I'm finding on the Web have been allowed the parent_id to be NULL and then use a trig开发者_如何转开发ger to update it. I'd rather update in one shot, if possible.
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)
And the following even simpler seems to work as well:
insert into test values(default, lastval());
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".
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)
The main question is - why would you want to insert record which relates to itself?
Schema looks like standard adjacency list - one of methods to implement trees in relational database.
The thing is that in most cases you simply have parent_id NULL for top-level element. This is actually much simpler to handle.
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