如何加入空值？

Question

此代码仅返回具有评级的帖子。如果帖子没有评级，我如何获取所有具有空值的帖子？

select 
    posts.id, 
    posts.user_id, 
    posts.name, 
    posts.created_at,
    ratings.post_id,
    ratings.avg,
    ratings.count
from 
    posts
join (
    select
        post_id,
        avg(rating) as avg, 
        count(rating) as count
    from 
        ratings
    group by 
        ratings.post_id
) ratings
on ratings.post_id = posts.id

Answer 1

你试过使用LEFT OUTER JOIN吗？ALEFT OUTER JOIN将保留join语句（posts表）“左侧”的所有行，这些行在语句（表）的“右侧”没有相应的行ratings。“右侧”缺少的行将用NULLS填充。