


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:
(SELECT users.username AS posted_by, users.id AS posted_by_id FROM users WHERE users.id = posts.posted_by)
This subquery retrieves two columns, "posted_by" and "posted_by_id". However, you use this subquery as a single column in the outer query:
COUNT( posts.solved_post ) AS solved_post, (SUBQUERY)
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.
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
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:
(SELECT users.username AS posted_by FROM users WHERE users.id = posts.posted_by)
By selecting a single column in a subquery, you ensure that it meets the single column requirements of the outer query.
The above is the detailed content of Why Does My MySQL Query Return an 'Operand should contain 1 column(s)' Error?. For more information, please follow other related articles on the PHP Chinese website!

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