Question

为了解释我尝试做什么，我会做出一个假设的情况。（忽略糟糕的数据库设计大声笑）

3张表：

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

我想生成一个查询，显示所有客户LEFT加入AM_Orders和PM_Orders，但我只希望 ONE TotalAmt字段，而不是2。

所以，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？

Answer 1

这有点含糊不清，但听起来你想要union all这两个结果：

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;

或作为Tab建议，先联合然后加入。对于一个表中有条目而另一个表中没有条目的情况，这可能会更好：

;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;

将2个字段组合成1个字段

1 个答案: