если столбец 2 пустой

(
    [CustomerID] [int] NOT NULL,
    [A] [int] NULL,
    [B] [int] NULL
)

select 
    CustomerID,
    case when A is null and B is null then 0 else 0 end as C,
    case when A is not null and B is not null then A else 0 end as A,
    case when A is not null and B is not null then B else 0 end as B
from Customers