在sql查询中获取零计数

这个话题以前已经讨论过了，但我仍然找不到解决办法。 我正在计算服务的开/关次数。 如果服务从未关闭过，例如id=7182，则该服务的表中不存在状态==“关闭”

如何获得计数==0，如下面的示例所示？ 我已经突出显示了我需要但无法显示的行的示例

提前多谢了

PS：我一直在玩coalesce，但是运气不好或者现在不知道如何正确使用它

SELECT id, status, count(*)
from history 
group by id, status
order by id, status; 

  id  | status | count 
------+--------+-------
 7182 | on     |    50
 7182 | off    |     0 <-- Not shown in the output as there is no id=7182 with status=off
 7183 | on     |    50
 7183 | off    |     0 <-- Not shown in the output as there is no id=7183 with status=off
 7184 | on     |    49
 7184 | off    |     1

---

如果我理解，现在不显示count=0，您希望在结果中得到它。试一试：

SELECT id, status, count(*) as c
from history 
group by id, status
having c >= 0
order by id, status;

---

一种方法是使用条件求和并将开/关计数作为

select
id,
sum( case when status = 'on' then 1 else 0 end)  as on_count,
sum( case when status = 'off' then 1 else 0 end ) as off_count
from history
group by id

---

表中缺少某些id/状态组合，但您希望查看它们。解决方案是使用交叉连接创建它们：

select i.id, s.status, count(*)
from (select distinct id from history) as i
cross join (select distinct status from history) as s
left join history h on h.id = i.id and h.status = s.status
group by i.id, s.status
order by i.id, s.status;

---

顺便说一句：当列ID不是表中记录的唯一ID时，命名列ID是个坏主意。

对不起，不清楚您需要哪个计数。你能再详细说明一下吗

select i.id, s.status, count(*)
from (select distinct id from history) as i
cross join (select distinct status from history) as s
left join history h on h.id = i.id and h.status = s.status
group by i.id, s.status
order by i.id, s.status;