How is your internal query execution?

challenge.sql

use blog;

-- Count posts id
SELECT COUNT(DISTINCT id) FROM posts;
-- => 7

-- count posts id
SELECT COUNT(DISTINCT post_id) from comments;
-- => 3

-- select all the posts that doesn't have comments
SELECT id FROM posts WHERE id NOT IN (SELECT DISTINCT post_id from comments);
-- => There are no results to be displayed.

-- hoooooow??? If I do this this works:
SELECT DISTINCT p.id
FROM posts p
         LEFT JOIN comments c ON c.post_id = p.id
WHERE c.id IS NULL
-- => [1, 2, 3, 4]

-- ping me for the solution or add your comments here

Your first one worked for me - http://sqlfiddle.com/#!9/1e130d/1

You are correct @gingerCodeNinja