SQL上的WHERE子句JOIN

Question

我有以下查询来返回客户使用的作业（访问次数）和他/她使用的独特的兽医数量。

SELECT 
customer.users_id AS CustID, 
COUNT(VisitID) AS Jobs, 
COUNT(DISTINCT VetID) AS Resources 
FROM customer 
LEFT JOIN visit ON customer.users_id = visit.CustID 
GROUP BY customer.users_id 

当我添加一个WHERE子句来获得只计算那些没有改期或取消访问，我的查询看起来是这样的。

SELECT 
customer.users_id AS CustID, 
COUNT(VisitID) AS Jobs, 
COUNT(DISTINCT VetID) AS Resources 
FROM customer LEFT JOIN visit ON customer.users_id = visit.CustID 
WHERE visit.Status != 'Cancelled' AND visit.Status != 'ReScheduled' 
GROUP BY customer.users_id 

然而，在这种情况下，我只得到，虽然我使用的是左连接，为客户谁没有任何取消或重新安排访问，埃本的客户。

我该如何适应WHERE子句？

Answer 1

你不想要WHERE条款。这只是将LEFT JOIN变成INNER JOIN。您在LEFT JOIN需要一个AND：

SELECT customer.users_id AS CustID, COUNT(VisitID) AS Jobs, COUNT(DISTINCT  VetID) AS Resources 
FROM customer 
LEFT JOIN visit ON customer.users_id = visit.CustID 
AND visit.Status NOT IN('Cancelled','ReScheduled') 
GROUP BY customer.users_id 

Answer 2

当我们添加一个where子句与left outer join，它像一个inner join，其中ON子句后应用的过滤器。与AND条款取代where

SELECT customer.users_id AS CustID, COUNT(VisitID) AS Jobs, COUNT(DISTINCT VetID) AS Resources 
FROM customer 
LEFT outer JOIN visit 
ON (customer.users_id = visit.CustID) 
AND visit.Status != 'Cancelled' 
AND visit.Status != 'ReScheduled' 
GROUP BY customer.users_id