Mysql - operand should contain 1 column(s)

While working on a system i'm creating i attempted to use the following query in my project

SELECT
topics.id,
topics.name,
topics.post_count,
topics.view_count,
COUNT( posts.solved_post ) AS solved_post,
(SELECT users.username AS posted_by,
    users.id AS posted_by_id
    FROM users
    WHERE users.id = posts.posted_by)
FROM topics
LEFT OUTER JOIN posts ON posts.topic_id = topics.id
WHERE topics.cat_id = :cat
GROUP BY topics.id

":cat" is bound by my PHP code as I'm using PDO. 2 is a valid value for ":cat".

That query though gives me an error: "#1241 - Operand should contain 1 column(s)"

What stumps me is that i would think this question would be perfect Selecting columns, then selecting two more from another table, and continuing on from there. I just don't know what the problem is

Is there any way to write a query?

Your subquery is selecting two columns, while you are using it to project one column (as part of the outer SELECT clause). You can select one column from such a query in this context

Consider joining to the users table instead; this will give you more flexibility when selecting what columns you want from users .

SELECT
topics.id,
topics.name,
topics.post_count,
topics.view_count,
COUNT( posts.solved_post ) AS solved_post,
users.username AS posted_by,
users.id AS posted_by_id

FROM topics

LEFT OUTER JOIN posts ON posts.topic_id = topics.id
LEFT OUTER JOIN users ON users.id = posts.posted_by

WHERE topics.cat_id = :cat
GROUP BY topics.id