ICE wrote:

...

klientów czy ich komputery?

chodzi oczywiscie o klientow,a nie o ich komputery ;)

spróbuj jakoś tak:

SELECT DISTINCT c.id, c.lastname, c.name, COALESCE(cash.balance, 0)

FROM customers c

JOIN customerassignments a ON (a.customerid = c.id) LEFT JOIN ( SELECT SUM(value) AS balance, customerid FROM cash GROUP BY customerid ) cash ON (cash.customerid = c.id) WHERE a.customergroupid IN (3,5,25) OR (a.customerid = 4 AND (cash.balance != 0 AND cash.balance IS NOT NULL))