Syntax help migrating MS SQL Server to Oracle

Hi all

I have the update statement written for MS SQL 2000 below and need to convert it to Oracle 9. It seems like Oracle does not support joins in update queries :(
How can I do this in Oracle 9?

Thanks in advance
Stefan

update t
set t.SRights = case when ((f1.Id is null and f2.Id is null) or f2.Id is not null) then 1 else 0 end
from GuiTmp t

       join ReadRights r on r.ObjectType  = t.ObjectType and
                               r.Status  = t.Status
       join Access_ a on r.Access_ = a.Access_
       join ObjTypeAcc oa on oa.Access_ = a.Access_ and
                                oa.ObjectType = r.ObjectType and
                                oa.Cfg = r.Cfg
       left join FieldAcc f1 on oa.Id = f1.ObjectTypeAccess and
                                   f1.Cfg = 1 and
                                   f1.Rights = 1 and
                                   f1.Field = t.Field and
                                   f1.Status = t.Status
       left join FieldAcc f2 on oa.Id = f2.ObjectTypeAccess and
                                   f2.Cfg = 1 and
                                   f2.Rights in (2,3) and
                                   f2.Field = t.Field and
                                   f2.Status = t.Status
where  r.Cfg = 1 and
       t.RRights = -1