MySQL - Operand should contain 1 column(s)

Question

Your subquery is selecting two columns, while you are using it to project one column (as part of the outer SELECT clause). You can only select one column from such a query in this context.

Consider joining to the users table instead; this will give you more flexibility when selecting what columns you want from users.

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

This error can also occur if you accidentally use commas instead of AND in the ON clause of a JOIN:

JOIN joined_table ON (joined_table.column = table.column, joined_table.column2 = table.column2)
                                                        ^
                                             should be AND, not a comma

This error can also occur if you accidentally use = instead of IN in the WHERE clause:

FOR EXAMPLE:

WHERE product_id = (1,2,3);

In my case, the problem was that I sorrounded my columns selection with parenthesis by mistake:

SELECT (p.column1, p.column2, p.column3) FROM table1 p WHERE p.column1 = 1;

And has to be:

SELECT p.column1, p.column2, p.column3 FROM table1 p WHERE p.column1 = 1;

Sounds silly, but it was causing this error and it took some time to figure it out.

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)

Well, you can’t get multiple columns from one subquery like that. Luckily, the second column is already posts.posted_by! So:

SELECT
topics.id,
topics.name,
topics.post_count,
topics.view_count,
posts.posted_by
COUNT( posts.solved_post ) AS solved_post,
(SELECT users.username AS posted_by_username
    FROM users
    WHERE users.id = posts.posted_by)
...

This error can also occur if you accidentally miss if function name.

for example:

set v_filter_value = 100;

select
    f_id,
    f_sale_value
from
    t_seller
where
    f_id = 5
    and (v_filter_value <> 0, f_sale_value = v_filter_value, true);

Got this problem when I missed putting if in the if function!

I got this error while executing a MySQL script in an Intellij console, because of adding brackets in the wrong place:

WRONG:

SELECT user.id
FROM user
WHERE id IN (:ids); # Do not put brackets around list argument

RIGHT:

SELECT user.id
FROM user
WHERE id IN :ids; # No brackets is correct