这是一个简单的JOIN.

尝试一下:

select c.comment_id,
    c.comment_content,
    u.user_id,
    u.user_name,
    c.post_id
from comments c
join users u on u.user_id = c.user_id;

如果您还需要发件表中的列，请加入它:

If you need columns from posts table too, join it:

select c.comment_id,
    c.comment_content,
    u.user_id,
    u.user_name,
    p.post_id,
    p.post_content
from comments c
join users u on u.user_id = c.user_id
join posts p on c.post_id = p.post_id;