How to Fix the 'Operand should contain 1 column(s)' MySQL Error?

MySQL error "operand should contain 1 column" solution

This article will solve the "operand should contain 1 column(s)" error that occurs in MySQL queries. This error usually occurs when a subquery returns multiple columns, but the outer query only expects a single column.

Misunderstanding:

The "operand should contain 1 column(s)" error indicates that the operand (here, a subquery) expected to return a single column, but actually returned multiple columns, resulting in a column number mismatch.

Problem Solved:

This problem can be solved by modifying the query to ensure that the subquery only returns a single column. There are two main methods:

1. Use JOIN:

Avoid using subqueries and directly use JOIN to join the users table with the posts table on the posted_by column. This method is more flexible when selecting specific columns in the users table.

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

2. Select a single column from a subquery:

Alternatively, you can modify the subquery to return only a single column using the GROUP BY clause. However, this approach may result in data loss if multiple users have the same ID.

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

Which method you choose depends on your specific needs and data structure. The first approach (using JOIN) is usually the more efficient and clearer solution.

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