How do you sum the same products in an invoice?

I have this query:

我有这个问题:

declare @usuario as varchar(50), @fecha as datetime
set @usuario ='angel'
set @fecha ='2011/08/07'

select DetalleCompra .id_ali  as [Clave],Alimento .nomAli as [Nombre Alimento], 
    (select sum (cantidad) 
     from DetalleCompra 
     inner join Alimento on DetalleCompra .id_ali =Alimento .id_ali 
     where DetalleCompra .usuario =@usuario  and fecha =@fecha  and FolioCompra is null
    ) as Cantidad, 
    Alimento .precio as [Precio Unitario], ( sum (Cantidad) * precio ) as [Precio Total] 
from DetalleCompra , Alimento 
where DetalleCompra .id_ali =Alimento.id_ali 
    and usuario =@usuario  and  fecha =CONVERT(datetime, @fecha ) and FolioCompra  is null
group by DetalleCompra .id_ali ,Alimento .nomAli , Alimento .precio , DetalleCompra .Cantidad  
declare @usuario