Include zero count in groupby - postgresql

Question

I have this table
person outcome
Peter positive
Peter positive
Peter positive
Eric positive
Eric positive
Eric negative
and want to count the number of rows each person has a positive/negative outcome.
select person, outcome, count(*)
from public.test123
group by person, outcome
person outcome count
Peter positive 3
Eric positive 2
Eric negative 1
But I also want a zero count for Peter negative. I've seen answers like this but I have nothing to join the table to?
How can I groupby, count and include zeros?
person outcome count
Peter positive 3
Peter negative 0
Eric positive 2
Eric negative 1
zxc
create table public.test123 (
person VARCHAR(20),
outcome VARCHAR(20));
insert into public.test123(person, outcome)
VALUES
('Peter', 'positive'),
('Peter', 'positive'),
('Peter', 'positive'),
('Eric', 'positive'),
('Eric', 'positive'),
('Eric', 'negative');

Answer 1

step-by-step demo:db<>fiddle
SELECT
s.person,
s.outcome,
SUM((t.outcome IS NOT NULL)::int) as cnt -- 4
FROM (
SELECT
*
FROM unnest(ARRAY['positive', 'negative']) as x(outcome), -- 1
(
SELECT DISTINCT -- 2
person
FROM test123
) s
) s
LEFT JOIN test123 t ON t.person = s.person AND t.outcome = s.outcome -- 3
GROUP BY s.person, s.outcome
Create a list of all possible outcome values.
Join it with all possible person values. Now you have a cartesian table with all possible combinations.
This can be used to join your original table.
Count all non-NULL values for each combination (in that case I uses SUM() with all non-NULL values == 1, 0 else)