Question

I have table A(id).

I need to

• create table B(id)
• add a foreign key to table A that references to B.id
• for every row in A, insert a row in B and update A.b_id with the newly inserted row in B

Is it possible to do it without adding a temporary column in B that refers to A? The below does work, but I’d rather not have to make a temporary column.

alter table B add column ref_id integer references(A.id);
insert into B (ref_id) select id from A;
update A set b_id = B.id from B where B.ref_id = A.id;
alter table B drop column ref_id;

Answer 1

Assuming that:

1) you’re using postgresql 9.1

2) B.id is a serial (so actually an int with a default value of nextval(‘b_id_seq’)

3) when inserting to B, you actually add other fields from A otherwise the insert is useless

…I think something like this would work:

with n as (select nextval('b_id_seq') as newbid,a.id as a_id  from a),
   l as (insert into b(id) select newbid from n returning id as b_id)
 update a set b_id=l.b_id from l,n where a.id=n.a_id and l.b_id=n.newbid;