MySQL - SELECT … (Subquery) AS fieldname

Question

I currently have one MySQL table of posts for a forum. This includes parent posts and child posts. If it is a parent post, it has a topic . If it is a child post, it includes a parent_id .

Eg:

TABLE posts

+----+-----------+---------+--------+------+
| id | parent_id |  topic  | author | body |
+----+-----------+---------+--------+------+
|  1 |      NULL | "Hello" |  "Me"  | ...  |
|  2 |         1 |  NULL   |  "Me"  | ...  |
+----+-----------+---------+--------+------+

I would like to run one SQL query, similar to the following:

SELECT id, 'self' AS parent_id, topic, author, body FROM posts WHERE parent_id IS NULL,
UNION
SELECT id, parent_id, (SELECT topic WHERE id=parent_id) as topic FROM posts WHERE topic IS NULL

Desired output:

+----+-----------+---------+--------+------+
| id | parent_id |  topic  | author | body |
+----+-----------+---------+--------+------+
|  1 |    "self" | "Hello" |  "Me"  | ...  |
|  2 |         1 | "Hello" |  "Me"  | ...  |
+----+-----------+---------+--------+------+

Basically, I want to return the parent topic without having to run multiple queries. Is there any way to do this? Should I just add a "parent_topic" field instead?

Thanks!

Answer 1

The following SQL will do what you're looking for:

select p_child.id, if(p_child.parent_id is null, 'self', p_child.parent_id) as parent_id, if(p_parent.id is null, p_child.topic, p_parent.topic) as topic, p_child.author, p_child.body
from posts p_child
    left join posts p_parent on p_child.parent_id = p_parent.id;

Answer 2

Try this query, if the parent_id is not null , I am selecting the parents' topic in a sub-query. I am using case to do that.

SELECT id,
    CASE WHEN parent_id IS NULL THEN 'SELF'
         ELSE parent_id
    END as parent_id
    CASE WHEN parent_id IS NOT NULL THEN (SELECT t.topic FROM posts as t WHERE t.id = m.parenyt_id)
        ELSE topic
    END as Topic,
    author,
    body
FROM posts as m