Combine two select queries from one table in MySQL

Question

I have mysql table with structure like

id | first_id | second_id
 1 |        1 |         2
 2 |        1 |         4
 3 |        1 |         9
 4 |        4 |         6
 5 |        5 |         9
 6 |        9 |        10

I want to get list of ids that are in one pair with x :

Each pair of ids represents relationship between 2 objects. So I want to get all ids of objects that have relations with object x .

SELECT first_id 
  FROM Table 
 where second_id = x 
  JOIN SELECT second_id 
         FROM Table 
        where first_id = x

This query return syntax error for some reason - what am I doing wrong?

Answer 1

The below query will check the x value with both second_id as well as first_id and get the first_id and second_id respectively.

SELECT first_id as Id
FROM Table 
WHERE second_id = x 
UNION ALL 
SELECT second_id 
FROM Table 
WHERE first_id = x

Answer 2

It should be something like:

SELECT 
    a.`id` as `id`,
    a.`first_id` as `first_id`,
    b.`first_id` as `related_id`
FROM Table a
JOIN Table b
ON a.`first_id` = b.`second_id`
WHERE a.`first_id` = x