UPDATE "Elektrarne" SET "Cena" =
(SELECT
CASE WHEN sum(z."Cena" * k."Pocet")::money IS NULL THEN '0'::money ELSE  sum(z."Cena" * k."Pocet")::money END
FROM
"Elektrarne" AS el
LEFT JOIN
"Komponenty" AS k
ON
k."ID_Elektraren"=el."Cislo"
LEFT JOIN
"Zariadenia" AS z
ON  z."ID_zariadenia" = k."ID_Zariadenia"
WHERE el."Cislo" = elektrarne."Cislo"
GROUP BY el."Cislo"
);