SQL：获取所有评论的所有帖子（包括0）

Question

此刻我有这个疑问：

select p.idpost, count(c.idpost)
from posts p join comments c on p.idpost = c.idpost
group by p.idpost

结果仅显示包含评论的帖子。我怎样才能获得所有帖子？

Answer 1

使用left join代替join

select p.idpost, count(c.idpost)
from posts p 
    left join comments c on p.idpost = c.idpost
group by p.idpost

Here is a good visual explanation of SQL joins with code它显示每次加入会发生什么

Answer 2

使用LEFT JOIN：

select
    p.idpost,
    count(c.idpost)
from posts p
LEFT join comments c on p.idpost = c.idpost
group by p.idpost

干杯