How to Fix the 'Operand Should Contain 1 Column' Error in MySQL's NOT IN Subquery?

MySQL "Operand Should Contain 1 Column" Error in NOT IN Subqueries: A Solution

MySQL's NOT IN clause requires the subquery to return a single column. The following query generates the "Operand should contain 1 column" error because the subquery returns multiple columns:

<code class="language-sql">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  
FROM campaigns d
LEFT JOIN served e
ON d.id = e.id_campaign 
WHERE 
    d.status = 'Active'
GROUP BY e.id_campaign
HAVING
    countcap < d.frequency)</code>

The problem lies in selecting multiple fields (e.g., e.id_campaign, d.name, d.frequency, etc.) within the subquery. NOT IN expects a single column for comparison.

The Solution: Using a Derived Table

To correct this, we can employ a derived table (a subquery in the FROM clause) to isolate the relevant column:

<code class="language-sql">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);</code>

This revised query uses a derived table named subquery. This inner query performs the original logic, but only selects e.id_campaign (aliased as id_campaign), ensuring the outer NOT IN clause receives a single-column result set, thus resolving the error. The original filtering logic based on frequency and countcap is retained within the derived table.

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