MySQL的自聯接返回空值

Question

我已經MySQL查詢以下

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select a.agency_name, a.green ,b.red, c.amber 
from (select count(action_status) as green, agency_name from tbl_actions 
where action_status='In Progress' group by agency_name) a 
join (select count(action_status) as red, agency_name from tbl_actions 
where action_status='Delayed' group by agency_name) b on a.agency_name=b.agency_name 
join (select count(action_status) as amber, agency_name from tbl_actions 
where act_status='Completed' group by agency_name) c on a.agency_name=c.agency_name 

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並且它返回空值 ​​

我有在數據庫中填充這些字段與相關數據。

Answer 1

條件聚合簡單得多：

select a.agency_name, 
     sum(a.action_status = 'In Progress') as green, 
     sum(a.action_status = 'Delayed') as red, 
     sum(a.action_status = 'completed') as amber 
from tbl_actions a 
group by a.agency_name; 

自聯接是沒有必要的。您在結果集中沒有行的原因是因爲沒有代理機構具有所有三種狀態。