SQL仅以一对多关系选择一条记录

Question

在下面的当前/潜在客户查询中，我需要显示CustomerID，客户的姓氏以及一列，该列显示客户是否已下达at least一个订单。 但是，正如预期的那样，如果客户下了多个订单（一对多关系），它将显示客户的多个记录。 问题 ：由于我们仅需要报告客户是否已下达至少一个订单，因此在这里如何仅显示每个客户一个记录？

SELECT c.customerID, o.OrderID, CASE When ISNULL(o.OrderID, 0) = 0 Then 0 Else 
1 End as YesNO
FROM Customers c
LEFT JOIN Orders o
 ON c.customerID = o.customerID

Answer 1

使用outer apply()

select 
    c.customerID
  , o.OrderID
  , case when o.OrderID is null then 0 else 1 end as YesNO
from Customers c
  outer apply (
    select top 1 o.OrderID
    from Orders o
    where c.customerID = o.customerID
  ) o

您也可以使用o.OrderId is null而不是ISNULL(o.OrderID, 0) = 0 。

---

使用group by和min()

select 
    c.customerID
  , min(o.OrderID) as OrderId
  , case when min(o.OrderID) is null then 0 else 1 end as YesNO
from Customers c
  left join Orders o 
    on c.customerID = o.customerID 
group by c.CustomerID  

Answer 2

使用分组依据。

SELECT c.customerID, o.OrderID, CASE When ISNULL(o.OrderID, 0) = 0 Then 0 Else 1 End as YesNOFROM Customers cLEFT JOIN Orders o ON c.customerID = o.customerID GROUP BY c.customerID

Answer 3

如果您相信问题的描述，即您想知道某个客户是否已下订单，但不需要为每个客户提供有代表性的OrderId ：

select C.CustomerId,
  case when exists ( select 42 from Orders as O where O.CustomerId = C.CustomerId )
    then 1 else 0 end as YesNo
  from Customers as C;

请注意，当您不需要确切的数字时， exists比count更有效。