Function to select existing value or insert new row - sql

Question

I'm new to working with PL/pgSQL, and I'm attempting to create a function that will either find the ID of an existing row, or will insert a new row if it is not found, and return the new ID.
The query contained in the function below works fine on its own, and the function gets created fine. However, when I try to run it, I get an error stating "ERROR: column reference "id" is ambiguous". Can anybody identify my problem, or suggest a more appropriate way to do this?
create or replace function sp_get_insert_company(
in company_name varchar(100)
)
returns table (id int)
as $$
begin
with s as (
select
id
from
companies
where name = company_name
),
i as (
insert into companies (name)
select company_name
where not exists (select 1 from s)
returning id
)
select id
from i
union all
select id
from s;
end;
$$ language plpgsql;
This is how I call the function:
select sp_get_insert_company('TEST')
And this is the error that I get:
SQL Error [42702]: ERROR: column reference "id" is ambiguous
Detail: It could refer to either a PL/pgSQL variable or a table column.
Where: PL/pgSQL function sp_get_insert_company(character varying) line 3 at SQL statement

Answer 1

As the messages says, id is in there twice. Once in the queries, once in the table definition of the return type. Somehow this clashes.
Try qualifying the column expressions, everywhere.
...
with s as (
select
companies.id
from
companies
where name = company_name
),
i as (
insert into companies (name)
select company_name
where not exists (select 1 from s)
returning companies.id
)
select i.id
from i
union all
select s.id
from s;
...
By qualifying the column expression the DBMS does no longer confuse id of a table with the id in the return type definition.
The next problem will be, that your SELECT has no target. It will tell you to do a PERFORM instead. But I assume you want to return the results. Change the body to
...
RETURN QUERY (
with s as (
select
companies.id
from
companies
where name = company_name
),
i as (
insert into companies (name)
select company_name
where not exists (select 1 from s)
returning companies.id
)
select i.id
from i
union all
select s.id
from s);
...
to do so.

Answer 2

In the function you display there is no need for returns table (id int). It's supposed to always return exactly one integer ID. Simplify to RETURNS int. This also makes ERROR: column reference "id" is ambiguous go away, since we implicitly removed the OUT parameter id (visible in the whole function block).
How to return result of a SELECT inside a function in PostgreSQL?
There is also no need for LANGUAGE plpgsql. Could simply be LANGUAGE sql, then you wouldn't need to add RETURNS QUERY, either. So:
CREATE OR REPLACE FUNCTION sp_get_insert_company(_company_name text)
RETURNS int AS
$func$
WITH s as (
select c.id -- still good style to table-qualify all columns
from companies c
where c.name = _company_name
),
i as (
insert into companies (name)
select _company_name
where not exists (select 1 from s)
returning id
)
select s.id from s
union all
select i.id from i
LIMIT 1; -- to optimize performance
$func$ LANGUAGE sql;
Except that it still suffers from concurrency issues. Find a proper solution for your undisclosed version of Postgres in this closely related answer:
Is SELECT or INSERT in a function prone to race conditions?