Пожалуйста, попробуйте это также ..
SELECT BrID
,Customer_Id
,0 as Account_Id
,Customer_Name as [Customer name]
,NIC_No
FROM Customer
UNION ALL
select BrID
,Customer_Id
,Account_Id
,CONCAT(First_Name,' ',Last_Name) as [Customer name]
,NIC_No
FROM Operator