Question

我有3张桌子：

tblCustomer
CustomerID  CustomerName
   1         Customer 1
   2         Customer 2

tblOrder
OrderID CustomerID  OrderTypeID LoanNumber
1            1            1      98513542
2            1            1      71283527
3            1            1      10268541
4            1            1      61258965

tblOrderType
OrderTypeID    OrderTypeName
   1             Purchase
   2             Rental

现在，我希望返回CustomerID，CustomerName，OrderTypeName和OrderCount，其中OrderCount是客户每个订单中有多少订单。我正在使用以下查询：

SELECT tblCustomer.CustomerID, tblCustomer.customerName, tblOrderType.OrderTypeName, tblOrder.OrderID
FROM tblCustomer 
INNER JOIN tblOrder 
ON tblCustomer.CustomerID = tblOrder.CustomerID
INNER JOIN tblOrderType 
ON tblOrderType.OrderTypeID = tblOrder.OrderTypeID

有点有效。除了OrderCount之外，显然我得到了所有要求。结果是这样的：

CustomerID         CustomerName      OrderTypeName     OrderID
    1               Customer 1         Purchase           1
    1               Customer 1         Purchase           2
    1               Customer 1         Purchase           3
    1               Customer 1         Purchase           4

但我正在寻找的是：

CustomerID         CustomerName      OrderTypeName     OrderCount
    1               Customer 1         Purchase             4

现在，我尝试将Count()添加到各个地方的查询中（Count(tblOrder.OrderID)一个），我收到tblCustomer.CustomerID is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause的错误。

这不是家庭作业。我只是不太了解sql和数据库的相互作用，因为它没有在我的学校里教过，而且我有一个朋友在向我投掷场景。

Answer 1

您需要使用分组和聚合：

SELECT tblCustomer.CustomerID, tblCustomer.customerName, tblOrderType.OrderTypeName, 
count (tblOrder.OrderID) asOrderCOunt
FROM tblCustomer 
INNER JOIN tblOrder 
ON tblCustomer.CustomerID = tblOrder.CustomerID
INNER JOIN tblOrderType 
ON tblOrderType.OrderTypeID = tblOrder.OrderTypeID
GROUP BY
tblCustomer.CustomerID, 
tblCustomer.customerName, 
tblOrderType.OrderTypeName

如果不对未聚合的所有内容进行分组，则无法使用聚合函数。

Answer 2

错误消息确实告诉您该问题。每当您使用聚合运算符（COUNT，SUM，AVG等）时，您必须告诉数据库应如何将它们聚合在一起。您使用GROUP BY语句执行此操作（SQL Server的链接，但它通常应用于所有数据库引擎）：

SELECT 
     C.CustomerID
    ,C.customerName
    ,OT.OrderTypeName
    ,COUNT(O.OrderID)
FROM tblCustomer C 
INNER JOIN tblOrder O 
   ON C.CustomerID = O.CustomerID
INNER JOIN tblOrderType OT
   ON OT.OrderTypeID = O.OrderTypeID
GROUP BY
     C.CustomerID
    ,C.customerName
    ,OT.OrderTypeName

Answer 3

试试这个，只需添加分组依据并计算分组行数

SELECT tblCustomer.CustomerID, tblCustomer.customerName, 
  tblOrderType.OrderTypeName, tblOrder.OrderID, 
  COUNT(tblOrder.OrderID) as OrderCount
FROM tblCustomer INNER JOIN tblOrder 
  ON tblCustomer.CustomerID = tblOrder.CustomerID
INNER JOIN tblOrderType 
  ON tblOrderType.OrderTypeID = tblOrder.OrderTypeID
GROUP BY tblCustomer.CustomerID, tblCustomer.customerName, 
  tblOrderType.OrderTypeName

如何获取计数并仅显示一行

3 个答案: