Zapytanie rekurencyjne z sumą w Postgresie

To jest uproszczona wersja poprawny @a_horse odpowiedz (po dyskusji z OP w komentarzach).
Działa z każdym (rozsądnie skończona) liczba poziomów w rekurencji.

Całkowita cena za podany 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;

Cena łączna za wszystkie projekty

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 tutaj (z poprawnym przypadkiem testowym)
_{Stary sqlfiddle}