postgres sql returns no row - sql

Question

I have 3 followings tables:
CREATE TABLE public.a
(
id bigint NOT NULL DEFAULT nextval('a_id_seq'::regclass),
CONSTRAINT a_pkey PRIMARY KEY (id)
)
CREATE TABLE public.b
(
id bigint NOT NULL DEFAULT nextval('b_id_seq'::regclass),
fkid bigint,
CONSTRAINT b_pkey PRIMARY KEY (id),
CONSTRAINT b_fkid_fkey FOREIGN KEY (fkid)
REFERENCES public.a (id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION
)
CREATE TABLE public.history
(
id bigint NOT NULL DEFAULT nextval('history_id_seq'::regclass),
CONSTRAINT history_pkey PRIMARY KEY (id)
)
As you can see, the table A has 1 to many relationship with table B. The history table keeps track of logical time inside the app. The following query:
WITH main_q AS (
SELECT
a.id
FROM a
LEFT JOIN b ON a.id = b.fkid
)
SELECT main_q.id,
max(history.id) as as_of
FROM main_q,history
GROUP BY
main_q.id
Returns no rows, even if tables A contains rows, because History table is empty. However, as soon as I add at least 1 row in history table, the query works.
Could someone please explain how I should modify my query so that it returns rows even if history is empty (as long as table A contains rows) ? I would expect to receive a NULL value in as_of column or 0. I tried COALESCE(max(history.id,0)) but this doesn't work either, well, because no rows in history
EDIT Example:
Table A content:
id
1
2
Table B content:
id, fkid
1, 2
2, 2
Table History content:
<empty>

Answer 1

The issue is the query below
SELECT main_q.id,
max(history.id) as as_of
FROM main_q,history
GROUP BY
main_q.id
Why not try a left join, you may have nothing to join, you can try something below
SELECT main_q.id,
max(history.id) as as_of
FROM main_q left join history on 1=1
GROUP BY
main_q.id

Answer 2

If I am not wrong, you are performing a cartesian product between main_q and history, because you haven't a predicate to join these 2 tables.
Likely, when you are performing this cartesian product, if the second table (history) is empty, the cartesian product returns nothing. As soon as you have 1 record in history, you will get data from Main_q. However, if you have more than 1 record in history, you will start to get duplicate data because of the cartesian product.