mySQL select occurrences in two columns given third colum value - mysql

Question

i have a table like this
Sender_id | Receiver_id | Topic
456 | 123 | 1
123 | 456 | 3
123 | 456 | 2
456 | 123 | 2
123 | 789 | 1
123 | 456 | 1
123 | 789 | 4
456 | 123 | 1
I want to know for every given Topic who are the speakers (can be both sender or receiver) involved in that conversation
eg. for Topic 2, Mr. 123 and Mr. 456 are the only two speakers
The desired result would be to have just the values: 123, 456
Speakers | Topic
123 | 2
456 | 2
this is my first attempt but i want ot merge the two resulting columns
SELECT *
FROM (
SELECT sender_id
FROM [table]
WHERE topic = '2'
AND sender_id !=0
GROUP BY sender_id) a
INNER JOIN (
SELECT receiver_id
FROM [table]
WHERE topic = '2'
AND receiver_id !=0
GROUP BY receiver_id) b
ON sender_id

Answer 1

You don't need a JOIN, you need a UNION:
SELECT sender_id Speakers
FROM [table]
WHERE topic = '2'
AND sender_id !=0
UNION
SELECT receiver_id
FROM [table]
WHERE topic = '2'
AND receiver_id !=0
You don't need to group to remove duplicate values as the UNION does that for you.

Answer 2

SELECT Sender_id as speakers, Topic from Table1 WHERE Topic ='2'
UNION
SELECT Receiver_id as speakers, Topic FROM Table1 WHERE Topic = '2'
Working fiddle http://sqlfiddle.com/#!9/4cf5e3/2

Answer 3

DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table
(id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,Sender_id INT NOT NULL
,Receiver_id INT NOT NULL
,Topic INT NOT NULL
);
INSERT INTO my_table (sender_id,receiver_id,topic) VALUES
(456,123,1),
(123,456,3),
(123,456,2),
(456,123,2),
(123,789,1),
(123,456,1),
(123,789,4),
(456,123,1);
SELECT speaker FROM
(
SELECT sender_id speaker,topic FROM my_table
UNION
SELECT receiver_id,topic FROM my_table
) x
WHERE topic = 2;
+---------+
| speaker |
+---------+
| 123 |
| 456 |
+---------+