probablemente estés buscando
para FULL OUTER JOIN
SELECT
coalesce(a.code,b.code),
coalesce(a."year",b.year),
coalesce(a.nb_a,0),
coalesce(b.nb_b,0),
coalesce(a.nb_a,0) + coalesce(b.nb_b,0) AS total
FROM table_a a full outer join table_b b on a.code = b.code and a.year = b.year;
coalesce | coalesce | coalesce | coalesce | total
----------+----------+----------+----------+-------
1 | 2013 | 0 | 1 | 1
1 | 2014 | 0 | 1 | 1
1 | 2017 | 1 | 0 | 1
2 | 2012 | 2 | 1 | 3
3 | 2014 | 2 | 0 | 2
(5 rows)
