Sql 将2个字段合并为1个字段

为了解释我要做的事，我会假设一个情况。（忽略坏的数据库设计lol）

3个表：

Customers: CustomerID (pk), CustomerName
AM_Orders: OrderID (pk), OrderDateTime, TotalAmt, CustomerID (fk), OrderType
PM_Orders: OrderID (pk), OrderDateTime, TotalAmt, CustomerID (fk) OrderType

我希望生成一个查询，显示所有加入AM_订单和PM_订单的客户，但我只希望有一个TotalAmt字段，而不是两个

因此，结果可能是

 ID  CName   TotalAmt OrderType
    1   Bob     $100     "AM"
    1   Bob     $10      "AM"
    3   Steve   $20      "PM"
    4   Tom     $20.45   "AM"
    5   Andy    $21      "PM"
    6   Jimmy    

---

我该怎么做

这有点含糊不清，但听起来像是要将所有结果联合起来：

select
    c.CustomerID ID,
    c.CustomerName Cname,
    o.TotalAmt,
    o.OrderType
from
    Customers c
        left join
    AM_Orders o
        on c.CustomerID = o.CustomerID
union all    
select
    c.CustomerID ID,
    c.CustomerName Cname,
    o.TotalAmt,
    o.OrderType
from
    Customers c
        left join
    PM_Orders o
        on c.CustomerID = o.CustomerID
order by
    ID;

或者按照Tab的建议，先联合，然后加入。这可能更好地处理一个表中有条目而另一个表中没有条目的情况：

;with all_orders as (
    select
        o.CustomerID,
        o.TotalAmt,
        o.OrderType
    from
        AM_Orders o
    union all
    select
        o.CustomerID,
        o.TotalAmt,
        o.OrderType
    from
        PM_Orders o
) select
    c.CustomerID ID,
    c.CustomerName Cname,
    a.TotalAmt,
    a.OrderType
from
    Customers c
        left join
    all_orders a
        on c.CustomerID = a.CustomerID
order by
    ID;

---

您可以左键联接到一个派生表，该派生表是两个Orders表的并集。