select syntax with condition

[Remi Fritzen]

Hi everybody
here is the query that I wanna modify :

SELECT sum(t.Cost) AS Cost, sum(t.Duration) AS Duration,p.*, c.Name AS ClientName, u.FirstName, u.LastName, u.EmailAddress
FROM tblProjects p
INNER JOIN tblClients c ON c.ID = p.ClientID
LEFT JOIN tblUsers u ON u.ID = p.Owner
LEFT JOIN tblTasks t ON t.ProjectID = p.ID
WHERE p.ID = '83' GROUP BY p.ID

I'd like add to add to the sum for the duration only when the cost is 0, and keep the sum of the cost like that :

so
ID Cost Duration ....
1 0
2 0
3 30