摘要：
Select字句在逻辑上是SQL语句最后进行处理的最后一步，所以，以下查询会发生错误：SELECT YEAR(OrderDate) AS OrderYear, COUNT(DISTINCT CustomerID) AS NumCustsFROM dbo.OrdersGROUP BY OrderYear;因为group by是在Select之前进行的，那个时候orderYear这个列并没有形成。如果要查询成功，可以像下面进行修改：SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCustsFROM (SELECT YEAR(OrderDate) 阅读全文
