# Number of Comments per Post LeetCode Solution

This Leetcode problem Number of Comments per Post LeetCode Solution is done in SQL.

# List of all LeetCode Solution

## Problem Statement

There is no primary key for this table, it may have duplicate rows.
Each row can be a post or comment on the post.
parent_id is null for posts. parent_id for comments is sub_id for another post in the table.

Write an SQL query to find number of comments per each post. Result table should contain post_id and its corresponding number_of_comments, and must be sorted by post_id in ascending order.

Submissions may contain duplicate comments. You should count the number of unique comments per post.
Submissions may contain duplicate posts. You should treat them as one post.

The result format is in the following example.

Example 1:
Input:

Output:

Explanation:
The post with id 1 has three comments in the table with id 3, 4 and 9. The
comment with id 3 is repeated in the table, we counted it only once.
The post with id 2 has two comments in the table with id 5 and 10.
The post with id 12 has no comments in the table.
The comment with id 6 is a comment on a deleted post with id 7 so we ignored it.

## Number of Comments per Post LeetCode Solution MySQL

``````select
t.post_id,
count(distinct s.sub_id) as number_of_comments
from
(
select
distinct sub_id as post_id
from
Submissions
where
parent_id is null
) as t
left join Submissions as s on t.post_id = s.parent_id
group by
t.post_id
order by
t.post_id;```Code language: SQL (Structured Query Language) (sql)```

## Number of Comments per Post LeetCode Solution MySQL (Another approach)

``````select
post_id,
from
(
select
distinct sub_id as post_id
from
Submissions
where
parent_id is null
) as s1
left join (
select
parent_id,
from
(
select
distinct sub_id,
parent_id
from
Submissions
) as ds
where
parent_id is not null
group by
parent_id
) as s2 on s1.post_id = s2.parent_id
order by
post_id;```Code language: SQL (Structured Query Language) (sql)```
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