[hach]

Código SQL [-]
select a_codigo, sum(a_cantidad) from a
where a_borrado=0 and a_codigo not in
(select a_codigo from b where b_borrado=0)
group by a_codigo
union
select a_codigo, sum (b_cantidad_real) from b
where b_borrado=0
group by a_codigo

Espero te sirva!!!

Saludos