How to insert row in table with foreign key to itself? - sql

Question

I have table that has foreign key for itself. Column parentid is foreign key and it cannot be NULL.
if I doINSERT INTO mytable(name) VALUES('name'), so it says that can't insert NULL to parentid. BUT, what value I can set to it if no row was inserted yet?!
How I can write script that will add row to this table?
Thank you

Answer 1

Remove the NOT NULL constraint, as it is an inappropriate constraint. If you do not have a ParentId then the value is NULL and should be allowed. Creating a dummy row just to have a dummy parentid creates unnecessary dependencies.

Answer 2

A trick: Have a dummy row with a dummy key, say 99999. Insert with this as the FK, and then change the FK to its real value. And do it in a transaction.

Answer 3

Disable the FK in charge.
Then do the insert
Then do an update with the new (generated?) PK-ID into the Self-FK-Field
Then Enable the FK back.
LIke so:
ALTER TABLE [Client] NOCHECK CONSTRAINT [FK_Client_MainClient]
INSERT INTO Client VALUES ...
#ClientID = SCOPE_IDENTITY()
IF #IsMainClient=1
BEGIN
UPDATE [Client]
SET MainClientID = #ClientID
WHERE ClientID = #ClientID
END
ALTER TABLE [Relatie] WITH CHECK CHECK CONSTRAINT [FK_Relatie_Relatie]

Answer 4

How to make a dummy row with both id and parentid equal to -1:
CREATE TABLE mytable(
id int NOT NULL IDENTITY,
parentid int NOT NULL,
PRIMARY KEY (id),
FOREIGN KEY (parentid) REFERENCES mytable(id)
) ;
SET IDENTITY_INSERT mytable ON ; <-- this allows you to insert the
INSERT INTO mytable(id, parentid) <-- auto incremented identity field
VALUES (-1, -1);
SET IDENTITY_INSERT mytable OFF ;
SELECT * FROM mytable ;
| id | parentid |
| -1 | -1 |
If you have many data from other tables that you want to transfer into this table, you can set the IDENTITY_INSERT variable to ON, insert the data and then set it to OFF again.
But as others said, it might be better to just remove the NOT NULL constraint from the parentid field.

Answer 5

You can alter the column to allow null then set the fk to the new identity and enable not null again.
This should work, though maybe there is a better way
CREATE TABLE mytable
(
id int IDENTITY(1,1) primary key,
name varchar(50) not null,
parentid int not null
)
go
alter table mytable
add constraint FK_mytable_parentid FOREIGN KEY ( parentid ) references mytable(id)
ALTER TABLE mytable alter column parentid int null;
insert into mytable(name)
select 'test'
update mytable
set parentid = SCOPE_IDENTITY()
where id = SCOPE_IDENTITY()
ALTER TABLE mytable alter column parentid int not null;
select * from mytable
drop table mytable

Answer 6

From what I understood you already have id before inserting and you can't insert it because identity field isn't letting you to.
Like you mentioned in your comment:
in 1 table I have the rows 34 'name1'
34, 35 'name2' 34 (id,name,parentid)
and I want to copy them to other table
First table
create table Table1
(
id int not null primary key,
name varchar(20) not null,
parentId int not null
)
insert Table1
values
(34, 'name1', 34),
(35, 'name2', 34)
Second table:
create table Table2
(
id int identity(1, 1) primary key,
name varchar(20) not null,
parentId int not null foreign key references Table2(id)
)
Copying data from the first table to the second one:
set identity_insert Table2 on
insert Table2(id, name, parentId)
select *
from Table1
set identity_insert Table2 on
[Update]
If the second table already has values then:
alter table Table2
add oldId int null
alter table Table2
alter column parentId int null
go
insert Table2(name, oldId)
select name, id
from Table1
update tt3
set parentId = tt2.id
from Table2 tt3
join Table1 tt1 on
tt1.id = tt3.oldId
join Table2 tt2 on
tt1.parentId = tt2.oldId
alter table Table2
drop column oldId
alter table Table2
alter column parentId int not null

Answer 7

Don't reference an IDENTITY column as a self-referencing foreign key. Use an alternative key of the table instead. I guess you are using IDENTITY as a surrogate key but every table ought to have a natural key as well, so the IDENTITY column shouldn't be the only key of your table.