Question

I have a table where all comments are stored. Some comments are questions, other – answers.
So, my table looks like this:

id  parent_id   message is_answer   answered
1   0           2x2     0           1
2   1           4       1           0
3   0           5x9     0           0
4   1           2x9     0           0
5   1           2x8     0           1
6   5           16      1           0

And as a result I’d like to get this table:

q_id    q_message   q_answer 
1       2x2         4
3       5x9         NULL 
4       2x9         NULL
5       2x8         16

So, I’d like to get all questions and answers to them, if they exist.

Answer 1

There is nothing that prevents you from joining a table to itself.

SELECT q.id AS q_id, q.message AS q_message, a.message AS q_answer
FROM table AS q
LEFT JOIN table AS a ON a.parent_id = q.id
WHERE q.is_answer = 0;

Note that this does not quite give the output you said you want… because it shows all answers to the question:

+------+-----------+----------+
| q_id | q_message | q_answer |
+------+-----------+----------+
|    1 | 2x2       | 4        | 
|    1 | 2x2       | 2x9      | 
|    1 | 2x2       | 2x8      | 
|    3 | 5x9       | NULL     | 
|    4 | 2x9       | NULL     | 
|    5 | 2x8       | 16       | 
+------+-----------+----------+
6 rows in set (0.00 sec)

Your requested output only shows the answer “4” for question 1. I’m not sure if that was intentional on your part–if so, you’ll need some additional GROUP BY, or other logic, to filter out the answers you do not want displayed.