Why Does My MySQL Query Return 'Operand should contain 1 column(s)' and How Can I Fix It?

MySQL query error: "Operand should contain 1 column"

Problem description:

While executing a MySQL query that joins multiple tables and contains a subquery (selecting columns from another table), an error occurred: "#1241 - Operand should contain 1 column(s)".

Query:

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

Error reason:

The

error occurs because the subquery selects two columns (users and username) from the id table, and the outer query expects the subquery to return only a single column.

Solution:

There are two ways to resolve this error:

1. Modify subquery:

   • Modify the subquery to return only username columns or id columns.

2. Connect users table:

   • Instead of using a subquery, join the posted_by table directly to the users table using the posts column. This approach provides more flexibility when selecting the required columns from the users table.

Modified query:

Use subquery:

<code class="language-sql">SELECT
topics.id,
topics.name,
topics.post_count,
topics.view_count,
COUNT( posts.solved_post ) AS solved_post,
(SELECT users.username AS posted_by
    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</code>

Use connection:

<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>

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