Question

我需要找到客户分别出现在订单和请求表中的次数。但是，此脚本为使用COUNT的两个位置生成相同的计数值。价值可能不一样，所以我做错了什么？

  SELECT   o.CustomerID, 
         COUNT(o.CustomerID) as OrdersPerCustomer,
         COUNT(r.CustomerID) as RequestsPerCustomer
  FROM Orders o
  INNER JOIN  [Customers] c on c.ID = o.CustomerID
  INNER JOIN  [Request] r on r.CustomerID = c.ID
  GROUP BY o.CustomerID

Answer 1

您将订单和请求记录的数量相乘。即通过加入表格，您可以获得3个订单和4个客户12个结果行的请求。由于ID在记录中永远不会为空，COUNT(o.CustomerID)和COUNT(r.CustomerID)只是COUNT(*)（在我的示例中为12，而不是预期的3和4）。

最简单的方法：

select
  customer_id,
  (select count(*) from orders o where o.customerid = c.id) as o_count,
  (select count(*) from request r where r.customerid = c.id) as r_count
from customers c;

与from子句中的子查询（派生表）相同：

select
  customer_id,
  coalesce(o.total, 0) as o_count,
  coalesce(r.total, 0) as r_count
from customers c
left join (select customerid, count(*) as total from orders group by customerid) o
  on o.customerid = c.id
left join (select customerid, count(*) as total from request group by customerid) r
  on r.customerid = c.id;

从多个表聚合时，始终先聚合然后加入。

在多个表中查找计数

1 个答案: