mysql 自连接返回空值

Question

我有以下 mysql 查询

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

它返回空白值作为

我在数据库中的这些字段中填充了相关数据。

Answer 1

条件聚合就简单多了：

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;

不需要自联接。您在结果集中没有得到任何行的原因是因为没有代理机构具有所有三种状态。

mysql 自连接返回空值

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