MySQL - 将一个表与一个主表连接两次

Question

我不确定这是否可以做到。但我只是想和这里的专家核实一下。

我的情况是： 我有一个表 tbl_campaign，它基本上存储了一个活动，该活动与一个名为 tbl_campaign_user 的表具有一对多关系，其中在活动期间选择的用户与活动 ID 一起存储（tbl_campagin_user.cu_campaign_id = tbl_campaign.campaign_id）。

第二个表 (tbl_campaign_user) 有一个状态字段，0 / 1 表示未发送/已发送。我想编写一个 sql 查询来读取活动数据以及已发送和未发送活动用户的数量（这就是我在第二个表上加入两次的原因）。 我在下面尝试了这个，但我得到的计数与已发送和未发送相同。

SELECT `tbl_campaign`.*, 
COUNT(sent.cu_id) as numsent, 
COUNT(unsent.cu_id) as num_unsent   FROM (`tbl_campaign`) 

LEFT JOIN tbl_campaign_user as sent on  (sent.cu_campaign_id = tbl_campaign.campaign_id and sent.cu_status='1')

LEFT JOIN tbl_campaign_user as unsent on  (unsent.cu_campaign_id = tbl_campaign.campaign_id and unsent.cu_status='0')


WHERE `tbl_campaign`.`campaign_id` = '19'  

---

我尝试通过将查询分成两部分进行调试： =>

SELECT `tbl_campaign`.*, 

COUNT(unsent.cu_id) as num_unsent   FROM (`tbl_campaign`) 

Left join tbl_campaign_user as unsent on  (unsent.cu_campaign_id = tbl_campaign.campaign_id and unsent.cu_status='0')

WHERE `tbl_campaign`.`campaign_id` = '19'

---

上面的工作完全符合要求。下面的也是这样：

=>

SELECT `tbl_campaign`.*, 
COUNT(sent.cu_id) as numsent   FROM (`tbl_campaign`) 

Left join tbl_campaign_user as sent on  (sent.cu_campaign_id = tbl_campaign.campaign_id and sent.cu_status='1')

WHERE `tbl_campaign`.`campaign_id` = '19' 

---

我不确定在合并两者时我做错了什么。我知道我对连接不太了解，所以可能是概念错误？请问谁能帮帮我？

提前谢谢！

Answer 1

您只需加入 tbl_campaign_user 一次， 计算（总和，随便）多少次 cu_status 为零/一。

SELECT `tbl_campaign`.id, 
count(u.id) as num_all_campaign_users
sum(u.cu_status) as num_sentcampaign_users, 
count(u.id) - sum(u.cu_status) as num_unsent_campaign_users   
FROM `tbl_campaign` c 
LEFT JOIN tbl_campaign_user as u on (u.cu_campaign_id = c.campaign_id)
WHERE `tbl_campaign`.`campaign_id` = '19'  
group by `tbl_campaign`.id

请注意，这是一种伪代码，您可能需要详细说明 select 子句和 group by 子句中的总和/计数。