Why Does My MySQL Query Return an 'Operand should contain 1 column(s)' Error?

MySQL error: fix "operand should contain 1 column" issue

In your MySQL query, you encountered the "Operand should contain 1 column" error. This error usually occurs when a subquery returns multiple columns, but the outer query only expects a single column at that location.

Understanding error:

The erroring query contains a subquery:

<code class="language-sql">(SELECT users.username AS posted_by,
    users.id AS posted_by_id
    FROM users
    WHERE users.id = posts.posted_by)</code>

This subquery retrieves two columns, "posted_by" and "posted_by_id". However, you use this subquery as a single column in the outer query:

<code class="language-sql">COUNT( posts.solved_post ) AS solved_post,
(SUBQUERY)</code>

This inconsistency results in the "operand should contain 1 column" error because the outer query expects a single column at this location.

Solution:

Method 1: Use table connection

To solve this problem, consider joining the "users" table directly instead of using a subquery. This will allow you to explicitly select the "posted_by" and "posted_by_id" columns without violating the single column constraint.

<code class="language-sql">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</code>

Method 2: Select a single column from a subquery

Alternatively, if you insist on using a subquery, you can modify it to only return the required columns. For example, you can retrieve only the "posted_by" column:

<code class="language-sql">(SELECT users.username AS posted_by
    FROM users
    WHERE users.id = posts.posted_by)</code>

By selecting a single column in a subquery, you ensure that it meets the single column requirements of the outer query.

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