Question

我多次尝试都没有成功。

问题是

列出2014年已下达2个或更多订单的安大略省客户的客户ID

我的回答是

SELECT Customer.CustId 
FROM Customer 
   INNER JOIN Orders ON Customer.CustId = Orders.CustId 
WHERE (Prov = 'ON' AND YEAR( Orders.OrderDate) = '2014') 
HAVING COUNT (Orders.CustId) > 2;

但是我一直收到一个错误，因为没有group by子句，但我不知道答案会在哪里发挥。

附件是物理架构

http://imgur.com/hbosBDM

Answer 1

HAVING仅与GROUP BY一起使用。只需包含COUNT（...）＆gt; 2作为WHERE子句的一部分。所以你的查询看起来像是：

SELECT Customer.CustId 
FROM Customer 
   INNER JOIN Orders ON Customer.CustId = Orders.CustId 
WHERE (Prov = 'ON' AND YEAR( Orders.OrderDate) = '2014') AND COUNT (Orders.CustId) > 2;

Answer 2

错误有意义，因为HAVING子句仅在分组（GROUP BY）时应用。应该起作用的查询版本如下：

SELECT Customer.CustId, COUNT(1) Cnt
FROM Customer C
   INNER JOIN Orders O ON C.CustId = O.CustId 
GROUP BY C.CustId
WHERE Prov = 'ON' AND YEAR(Orders.OrderDate) = 2014 
HAVING COUNT (1) > 2;

为了优化一点，您应该避免条件中的标量函数，因为该列上的任何索引都不能使用：

SELECT Customer.CustId, COUNT(1) Cnt
FROM Customer C
   INNER JOIN Orders O ON C.CustId = O.CustId 
GROUP BY C.CustId
WHERE Prov = 'ON' AND (Orders.OrderDate BETWEEN '20140101' AND '20141231') 
HAVING COUNT(1) > 2;

Answer 3

SELECT Customer.CustId 
FROM Customer 
   INNER JOIN Orders ON Customer.CustId = Orders.CustId 
WHERE (Prov = 'ON' AND YEAR(Orders.OrderDate) = 2014) 
GROUP BY Customer.CustId   --<-- you need to add this group by here 
HAVING COUNT (Orders.CustId) > 2;

SQL需要分组，但我不知道它是如何应用的

3 个答案: