如何在表中插入默认值的行？

Question

I have such table:

--
-- PostgreSQL database dump
--

SET statement_timeout = 0;
SET client_encoding = 'UTF8';
SET standard_conforming_strings = on;
SET check_function_bodies = false;
SET client_min_messages = warning;

SET search_path = public, pg_catalog;

SET default_tablespace = '';

SET default_with_oids = false;

--
-- Name: forum; Type: TABLE; Schema: public; Owner: postgres; Tablespace: 
--

CREATE TABLE forum (
    forum_id integer DEFAULT nextval('seq_forum'::regclass) NOT NULL,
    forum_name character varying NOT NULL,
    group_id integer NOT NULL,
    forum_parent integer DEFAULT (-1)
);


ALTER TABLE public.forum OWNER TO postgres;

--
-- Name: PK_forum; Type: CONSTRAINT; Schema: public; Owner: postgres; Tablespace: 
--

ALTER TABLE ONLY forum
    ADD CONSTRAINT "PK_forum" PRIMARY KEY (forum_id);


--
-- Name: FK_group; Type: FK CONSTRAINT; Schema: public; Owner: postgres
--

ALTER TABLE ONLY forum
    ADD CONSTRAINT "FK_group" FOREIGN KEY (group_id) REFERENCES groups(group_id) ON UPDATE CASCADE ON DELETE CASCADE;


--
-- Name: FK_parent; Type: FK CONSTRAINT; Schema: public; Owner: postgres
--

ALTER TABLE ONLY forum
    ADD CONSTRAINT "FK_parent" FOREIGN KEY (forum_parent) REFERENCES forum(forum_id);


--
-- PostgreSQL database dump complete
--

As you see above, this table has (at least should have ...) a default value in column forum_parent. I want to insert some data to this table, I do it like this:

INSERT INTO forum (forum_name, group_id) VALUES('new forum', 1); 

yup, I have a group with id = 1. But this code gives me:

PostgreSQL error: 23503 ERROR:  insert or update on table "forum" violates foreign key constraint "FK_parent"
DETAIL:  Key (forum_parent)=(-1) is not present in table "forum".

NOTICE:  there is no transaction in progress

How to make it right?

Answer 1

Your INSERT statement is correct. When you don't explicity declare the column name on your INSERT statement, the value that is inserted on the table is the default ( in your case, it's -1 ).

But the problem lies on the referential constraint. The column forum_parent of table forum is dependent on the values of column forum_id of the same table. As this DDL says,

ALTER TABLE ONLY forum
       ADD CONSTRAINT "FK_parent" FOREIGN KEY (forum_parent) 
       REFERENCES forum(forum_id);

The INSERT statement failed during execution because the value -1 is not present on column forum_id .

My suggestion is to change the default value from -1 to NULL

CREATE TABLE forum 
(
    forum_id integer DEFAULT nextval('seq_forum'::regclass) NOT NULL,
    forum_name character varying NOT NULL,
    group_id integer NOT NULL,
    forum_parent integer DEFAULT NULL
);

The difference between NULL and -1 is that NULL is simply unknow. or the value does not exist while -1 is an existing numeric value.

Answer 2

You added the following constraint:

ALTER TABLE ONLY forum
 ADD CONSTRAINT "FK_parent" FOREIGN KEY (forum_parent) REFERENCES forum(forum_id);

Which basically says that forum_parent must match an existing value of forum_id. You probably don't have a row with forum_id = -1 and therefore it fails.

You'll need to create a row with forum_id = -1 and only then can you use that default value...

Another option is to put the default value to be null since forum_parent is nullable