MySQL NOT IN Error: Why 'Operand Should Contain 1 Column'?

MySQL NOT IN query failed: operand should contain 1 column

When you encounter the "operand should contain 1 column" error in a MySQL NOT IN query, it's crucial to understand the root cause.

This error occurs because the operand of the NOT IN condition contains multiple columns:

SELECT * from campaigns WHERE id not in (
    SELECT 
        e.id_campaign, d.name, d.frequency, d.country, d.referral, d.bid, d.status, COUNT(e.id) AS countcap  

The NOT IN operator requires its operand to be a single column, while the subquery returns rows containing multiple columns.

Solution:

One way to solve this error is to extract the necessary columns from the subquery and compare it with the id column of the outer query:

select *
from campaigns 
where id not in (
    select id_campaign from (
        select e.id_campaign as id_campaign, d.frequency, e.id
        from campaigns d left join served e on d.id = e.id_campaign
        where d.status = 'Active'
        group by e.id_campaign 
        having count(e.id) < d.frequency 
    ) as subquery
)

Ensures correct use of the id_campaign operator by limiting the subquery results to a single column NOT IN. Additionally, we added an alias as subquery for the subquery, which is necessary in some MySQL versions. This improves query clarity and readability.

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