GROUP BY in MySQL query

Question

Suppose, I have a table like this:

+---------+------------------+
| post_id | original_post_id |
+---------+------------------+
| 1       | 0                |
| 2       | 0                |
| 3       | 0                |
| 4       | 2                |
| 5       | 2                |
| 6       | 0                |
| 7       | 1                |
| 8       | 1                |
| 9       | 1                |
| 10      | 0                |
| 11      | 0                |
| 12      | 3                |
+---------+------------------+

In a nutshell, in this table I contain users posts, and I'd like to show something like a timeline. All the records, which have original_post_id=0 - are the original posts, and must be shown in any case. But all the records with original_post_id!=0 - are the reposted, so I'd like to show them just one time. So, as a result, I need something like this:

+---------+------------------+
| post_id | original_post_id |
+---------+------------------+
| 1       | 0                |
| 2       | 0                |
| 3       | 0                |
| 4       | 2                |
| 6       | 0                |
| 7       | 1                |
| 10      | 0                |
| 11      | 0                |
| 12      | 3                |
+---------+------------------+

As I understand, I have to use GROUP BY, but how exactly?

Answer 1

You can do this using a single group by, but it is tricky:

select min(post_id) as post_id, min(original_post_id)
from t
group by original_post_id,
         (case when original_post_id = 0 then post_id end);

Answer 2

you can try this:

SELECT * FROM tableName
Where original_post_id = 0
Union
SELECT * FROM tableName
Where original_post_id != 0
Group by original_post_id