Question

表 - 我删除了查询不需要的其他字段：

church (id, name)
delegate (id, orderId, churchId, status)
orders (id, eventId)
event (id, name)

变量ddEvent在运行时包含事件名称（不是eventId）

我想要一份完整的教会名单列表，其中包含代表人数（如果有的话）进入特定活动，其状态为“付费”，“开始”和“开始”。或者＆＃39;已完成＆＃39;

我试过这个：

select church.name as 'Church Name', count(delegate.id) as Delegates
from church
left join delegate on church.id = delegate.churchId and (delegate.status = 'Paid' or delegate.status = 'Started' or delegate.status = 'Completed')
join orders on delegate.orderId = orders.id
join event on orders.eventId = event.id AND event.Name = 'ddEvent'
group by church.id

然而，这并没有产生正确的结果

Answer 1

尝试对2个相关表进行左连接。

select church.name as 'Church Name', count(delegate.id) as Delegates 
from church 
left join delegate on church.id = delegate.churchId 
and (delegate.status = 'Paid' or delegate.status = 'Started' or delegate.status = 'Completed') 
left join orders on delegate.orderId = orders.id 
left join event on orders.eventId = event.id
where event.Name = 'ddEvent' 
group by church.id

@Barmar谢谢你！

Answer 2

如果您需要计算参加某个活动的代表人数，那么您无法在代表之间进行左联接直至活动。这些连接需要是内连接。

您必须做的是围绕联接的顺序，确保delegate - orders - events表首先加入内部，然后将它们的组合结果保留在{ {1}}。您需要在church子句中使用括号来实现此目的：

from

请注意我如何将select church.name as 'Church Name', count(delegate.id) as Delegates from church left join (delegate inner join orders on delegate.orderId = orders.id inner join event on orders.eventId = event.id AND event.Name = 'ddEvent') on church.id = delegate.churchId and (delegate.status = 'Paid' or delegate.status = 'Started' or delegate.status = 'Completed') group by church.id子句加入on表和church表移动到delegate子句的末尾。

左边从不同的桌子加入计数

2 个答案: