表/连接/一张表和另一张表的聚合字段连接

用Northwind中的表：

SELECT CustomersID,CompanyName,ContactName,Address,
City,Region,Phone,a.orderCount
FROM dbo.CustomersInfo,(SELECT CustomerID,SUM(OrderAmount) AS orderCount FROM dbo.Orders GROUP BY CustomerID) as a
WHERE CustomersInfo.CustomersID=a.CustomerID

主要把select的结果集重新命名a

这样就可以调用a的字段了