mysql - 如何获取只有 1 名成员的群组数量？

Question

我正在努力

select count(*)
  from groups
  left join members on groups.id=members.group_id
 group by groups.id
having count(members.id)=1

但它只给了我一堆 1。如何获取行数？

Answer 1

只需将查询嵌入为子查询即可计算查询结果。

顺便说一句，内部联接比外部联接更适合您的查询。事实上，只有在 Members 表中存在无效的 group_id 值时才需要联接。

使用内连接:

select count(*)
from
(
  select groups.id
  from groups 
  inner join members on groups.id=members.group_id
  group by groups.id
  having count(members.id)=1
) as sub

没有加入(假设没有孤儿):

select count(*)
from
(
  select group_id
  from members
  group_id
  having count(*) = 1
) as sub