SQL需要分組，但我不知道它是如何應用的

Question

我已經嘗試了很多次而沒有成功。

的問題是

列出客戶ID爲已放置2個或更多的訂單在2014年

我的回答是

安大略客戶

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; 

但我不斷收到一個錯誤，因爲沒有group by子句，但我沒有得到在答案中播放的位置。

附件是物理模式

http://imgur.com/hbosBDM

Answer 1

HAVING僅在與GROUP BY結合使用。只需包含COUNT（...）> 2作爲WHERE子句的一部分即可。分組（GROUP BY）時

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

錯誤是有道理的，因爲HAVING子句僅適用於：所以您的查詢會是什麼樣子。應工作查詢的一個版本是這樣的：

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; 

要優化一點點，你應該避免在條件標量函數，因爲在該列任何索引無法使用：

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;