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

MySQL error: "Operation should contain 1 column" solution

In a recent SQL query, a developer encountered a confusing error: "The operand should contain 1 column." The error stopped their progress, but after careful investigation, a solution was found.

The query in question attempts to retrieve data from two tables, 'topics' and 'posts', and contains statistics for a subquery that selects two columns from the 'users' table. However, this subquery attempted to project two columns into one, resulting in the error message.

Understanding error:

The "operand should contain 1 column" error occurs when a query attempts to operate on multiple columns (e.g., aggregate) as if they were a single column. In this particular case, the subquery selects both 'username' and 'id' from the 'users' table.

Problem solved:

To resolve this issue, the subquery was reconfigured to select only one column by joining directly to the 'users' table. This approach allows more flexibility in selecting the required columns.

Rewritten query:

<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>

Advantages of direct connection:

Joining directly to the 'users' table provides several benefits:

• Flexibility: Selecting columns from the 'users' table becomes more straightforward and allows selecting only necessary data.
• Simplify subquery: Removing a subquery simplifies the entire query, making it easier to read and maintain.
• Improved performance: Directly joining tables can sometimes improve performance by reducing the number of queries required to retrieve the required data.

The above is the detailed content of Why Does My MySQL Query Return 'Operand should contain 1 column(s)'?. For more information, please follow other related articles on the PHP Chinese website!