如何将同一表上的两个查询连接在一起？

Question

I have a MySQL table, called related_post that looks like this:

+----+--------+--------+
| id | post_1 | post_2 |
+----+--------+--------+
|  1 |     25 |     26 |
|  2 |     25 |     27 |
|  3 |     25 |     28 |
|  4 |     26 |     27 |
|  5 |     26 |     28 |
|  6 |     27 |     28 |
|  … |     …  |     …  |
+----+--------+--------+

I need to retrieve all the related posts for a given post ID.

If I supply '27', I want it to return a single column with "25", "26", and "28" as values.

Right now, I need to split it up into 2 queries:

SELECT post_1 FROM related_post where post_2 = 27;
SELECT post_2 FROM related_post where post_1 = 27;

I would like to know if it's possible to perform only one query to accomplish the same result.

Answer 1

Try UNION :

SELECT post_1 AS post FROM related_post where post_2 = 27
UNION
SELECT post_2 FROM related_post where post_1 = 27

UNION will also remove duplicates; if you do not want this then use UNION ALL

Answer 2

you can use a CASE statement to do this in one query

SELECT CASE WHEN post_2 = 27 THEN post_1 ELSE post_2 END as post
FROM related_post 
WHERE post_2 = 27 OR post_1 = 27;