Count tables with the same ID in a column and display the highest [closed]

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I want to show the 5 threads with highest number of posts, but im kinda broken and I cant even imagine how to do it

the SQL tables are like this, of course is just an example

id | thread | subject | body
________________________________________________________
1  | NULL   | Thread1 | this is the body of the first thread id1
2  | 1      | NULL    | post id2 in the first thread id1
3  | NULL   | Thread2 | this is the body of the 2nd thread id3
4  | 2      | NULL    | post id4 in the second thread id4
5  | 2      | NULL    | post id4 in the second thread id5
6  | NULL   | Thread3 | this is the body of the 3rd thread id6
7  | 6      | NULL    | post id4 in the third thread id7
8  | 6      | NULL    | post id4 in the third thread id8
9  | 6      | NULL    | post id4 in the third thread id9

I want the result to be like this

thread3
thread2
thread1

I should do a separate query?, I mean 2 instead of only one, how?

Answer 1

You need to join the table to itself, with the main thread row in the first side of the join and the post rows in the other side:

select t1.subject
from mytable t1
join mytable t2 on t2.thread = t1.id
where t1.thread is null
group by 1
order by count(*) desc
limit 5