MYSQL get all posts grouped by date - mysql

Question

I want to get all posts in my database and have them returned, grouped by date (year -> month -> day -> posts for this day).
My current database structure:
ID, author, title, content, created_at
And possible values can be:
1, 1, "hello world", "This is the content", "2020-07-27 09:15:57"
2, 1, "Another post", "Another post content", "2020-07-27 11:09:55"
3, 1, "Third post", "No content", "2020-07-28 08:15:20"
...etc
What i want to achieve is, getting all these posts, but group them by date (year, month, day).
i.e.:
[2020] => [
[7] => [
[27] => [
[ID] => 1,
[ID] => 2,
],
[28] => [
[ID] => 3,
]
]
]
What i've tried so far:
SELECT id FROM my_posts_table WHERE author = 1 GROUP BY created_at ORDER BY created_at ASC
AND
SELECT id, YEAR(created_at) as year, MONTH(created_at) as month, DAY(created_at) as day FROM my_posts_table WHERE author = 1 GROUP BY year, month, day ORDER BY year, month, day ASC
My expected result would be the above example of what i want to achieve. But what i'm actually getting is:
[0] => [
[ID] => 1,
[year] => 2020,
[month] => 7,
[day] => 27
],
[1] => [
[ID] => 3,
[year] => 2020,
[month] => 7,
[day] => 28
]
I am completely confused at this moment because i thought 'GROUP BY' was working as i expected, but it appears to be working in a different way.
Can you guys point me in the right direction? I hope it's clear what i'm trying to achieve.
But if it's not, here is it again: "I want to get all the posts from my_posts_table where author_id = 1 and have them returned by year -> month -> day -> posts", So every post on day "27" should be under the [27] key..
EDIT: I made a SQLFiddle as requested

Answer 1

Use GROUP_CONCAT
For your SQLFiddle example try this:
SELECT GROUP_CONCAT(id),
YEAR(created_at) as year, MONTH(created_at) as month, DAY(created_at) as day
FROM my_posts_table
WHERE author = 1
GROUP BY year, month, day ORDER BY year, month, day ASC

Answer 2

A database query always returns a "flat" resultset, made of rows and columns. If you want some a nested datastructure, then one approach is to write a scalar query, that returns just one rows and one column containing a JSON object.
This would involve several levels of aggregation, and JSON functions JSON_ARRAYAGG() and JSON_OBJECT():
select json_object(created_year, json_arrayagg(obj)) obj
from (
select
created_year,
json_object(created_month, json_arrayagg(obj)) obj
from (
select
year(created_at) created_year,
month(created_at) created_month,
json_object(day(created_at), json_arrayagg(id)) obj
from my_post_table
group by year(created_at), month(created_at), day(created_at)
) t
group by created_year, created_month
) t