Това е опростена версия на правилно на @a_horse отговор
(след обсъждане с OP в коментарите).
Работи с всички (сравнително краен) брой нива в рекурсията.
Обща цена за даден project_id
WITH RECURSIVE cte AS (
SELECT project_id AS project_parent, project_value
FROM projects
WHERE project_id = 1 -- enter id of the base project here !
UNION ALL
SELECT p.project_id, p.project_value
FROM cte
JOIN projects p USING (project_parent)
)
SELECT sum(project_value) AS total_value
FROM cte;
Обща цена за всички проекти
WITH RECURSIVE cte AS (
SELECT project_id, project_id AS project_parent, project_value
FROM projects
WHERE project_parent IS NULL -- all base projects
UNION ALL
SELECT c.project_id, p.project_id, p.project_value
FROM cte c
JOIN projects p USING (project_parent)
)
SELECT project_id, sum(project_value) AS total_value
FROM cte
GROUP BY 1
ORDER BY 1;
db<>fiddle тук
(с правилен тест)
Стар sqlfiddle