从MySQL中的一个表合并两个选择查询

Question

我有结构像mysql表

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

我想获取与x成对的ID列表：

每对ID代表2个对象之间的关系。 因此，我想获取与对象x有关系的所有对象ID。

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

由于某种原因，该查询返回语法错误-我在做什么错？

Answer 1

下面的查询将同时检查second_id和first_id的x值，并分别获取first_id和second_id。

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

Answer 2

应该是这样的：

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