Postgres 12

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE results (user_id int, workout_id int, score int);
INSERT INTO results VALUES
  (1, 1, 10)
, (1, 3, 10)
, (1, 3, 15)
, (2, 1, 5)
;

CREATE TABLE



INSERT 0 4


SELECT DISTINCT ON (1, 2)
       user_id, workout_id, r.score
FROM        unnest('{1,2}'::int[])   u(user_id)
CROSS  JOIN unnest('{1,2,3}'::int[]) w(workout_id)
LEFT   JOIN results r USING (user_id, workout_id)
ORDER  BY 1, 2, 3 DESC NULLS LAST;


user_id
workout_id
score


1
1
10

1
2
null

1
3
15

2
1
5

2
2
null

2
3
null



SELECT 6


SELECT user_id, workout_id, max(r.score) AS score
FROM        unnest('{1,2}'::int[])   u(user_id)
CROSS  JOIN unnest('{1,2,3}'::int[]) w(workout_id)
LEFT   JOIN results r USING (user_id, workout_id)
GROUP  BY 1, 2
ORDER  BY 1, 2;


user_id
workout_id
score


1
1
10

1
2
null

1
3
15

2
1
5

2
2
null

2
3
null



SELECT 6

user_id	workout_id	score
1	1	10
1	2	null
1	3	15
2	1	5
2	2	null
2	3	null

user_id	workout_id	score
1	1	10
1	2	null
1	3	15
2	1	5
2	2	null
2	3	null