Question

如何计算event_participants表中的参与者数量，并显示event_title，结果为零。这是我的表

event_title
id | name  
1  | New York 
2  | Canada   


event_participants

id | event_title_id | name
1  | 1              | Jon
2  | 1              | Mike

这是我的查询

SELECT count( * ) AS count
FROM event_participants AS t1
LEFT JOIN event_title AS t2 ON t1.event_title_id = t2.id
GROUP BY t1.event_title_id

我得到了正确的结果，但没有显示0结果。我应该得到这样的结果

New York | 2
Canada   | 0

Answer 1

代替*使用t2.id它将起作用

SELECT count(t2.id) AS count
FROM event_participants AS t1
LEFT JOIN event_title AS t2 ON t1.event_title_id = t2.id
GROUP BY t1.event_title_id

Answer 2

你的小组是问题所在。您正在按event_title_id分组，其中您没有ID 2。

你应该做的是第一个表的name组。

SELECT count( * ) AS count
FROM event_participants AS t1
LEFT JOIN event_title AS t2 ON t1.event_title_id = t2.id
GROUP BY t2.name

应该有效

Answer 3

如下所示更改您的查询以获得所需的输出：

SELECT count(t2.id) AS count
FROM event_title AS t1
LEFT JOIN event_participants AS t2 ON t2.event_title_id = t1.id
GROUP BY t1.id

//To retrieve title also

SELECT t1.title,count(t2.id) AS count
FROM event_title AS t1
LEFT JOIN event_participants AS t2 ON t2.event_title_id = t1.id
GROUP BY t1.id

在左连接查询中显示0结果

3 个答案: