查询两个表，产生交集的数据，初学者很容易犯的错误。

		
SELECT 
    COUNT(n1_0.id)
FROM
    notification n1_0,
    notification_status ns1_0
WHERE
    n1_0.id <> ns1_0.notification_id	
		
			

正确的做法是使用 LEFT OUTER JOIN

		
SELECT 
    COUNT(n1_0.id)
FROM
    notification n1_0 LEFT OUTER JOIN notification_status ns1_0 ON n1_0.id<>ns1_0.notification_id;		
		
			

LEFT JOIN 关键字从左侧表返回所有的行，即使右侧表中没有匹配。如果右侧表中没有匹配，则结果为 NULL。

根据两个或多个表中的列之间的关系，从这些表中查询数据。

举例

			
SELECT 
    *
FROM
    notification n
        LEFT JOIN
    notification_status ns ON n.id = ns.notification_id
WHERE
    ns.notification_id IS NOT NULL;
			
			

使用 JOIN 替代 LEFT JOIN

			
SELECT 
    *
FROM
    notification n
        JOIN
    notification_status ns ON n.id = ns.notification_id;					
			
			

			
SELECT 
    *
FROM
    notification n
        INNER JOIN
    notification_status ns ON n.id = ns.notification_id;