Seria mas o menos algo asi
Código SQL
[-]
Select a.Codigo, Sum (A.Existent) As ExistenTT, B.Formula, b.Present, b.concentra
From Barras a
Left Join Productos B on A.Codigo = B.Codigo
Where b.Inventario = 'SI'
Group By codigo,b.formula,b.present,b.concentra
Having Sum (A.Existent) = 0
order by ExistenTT,codigo asc
comentas si te funciona