Why Does My MySQL `NOT IN` Query with `COUNT()` Result in an 'Operand Should Contain 1 Column' Error?

MySQL "Operand Should Contain 1 Column" Error in NOT IN Subqueries with Aggregate Functions

Using NOT IN subqueries in MySQL requires careful attention to column counts. A common error, "Operand should contain 1 column," arises when the subquery uses aggregate functions like COUNT(), returning multiple columns. MySQL's NOT IN operator expects a single-column comparison.

Root Cause:

The error stems from a mismatch in the number of columns returned by the main query and the subquery. The NOT IN subquery, containing an aggregate function, produces a result set with more than one column, conflicting with the single-column id in the main query's WHERE clause.

Illustrative Example:

Consider this problematic query:

<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 < p>The intention is to select campaigns *not* included in the subquery's results. The subquery, however, returns eight columns, causing the "Operand should contain 1 column" error because `NOT IN` expects a single-column comparison against the `id` column in the `campaigns` table.</p><p>**Resolution:**</p><p>The solution involves restructuring the subquery to return only the `id_campaign` column:</p><pre class="brush:php;toolbar:false"><code class="language-sql">SELECT *
FROM campaigns
WHERE id NOT IN
(
    SELECT e.id_campaign
    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) < 
</code>

Alternatively, for situations requiring multiple columns, use `EXISTS` or `NOT EXISTS` for a more efficient and accurate solution:

<code class="language-sql">SELECT *
FROM campaigns c
WHERE NOT EXISTS
(
    SELECT 1
    FROM campaigns d
    INNER JOIN served e ON d.id = e.id_campaign
    WHERE d.id = c.id
      AND d.status = 'Active'
      AND COUNT(e.id) < 
</code>

This revised approach avoids the column count mismatch and provides a cleaner solution for scenarios involving aggregate functions within subqueries used with `NOT IN`.

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