How to Fix MySQL's 'Invalid Use of Group Function' Error in a Subquery?

Debugging MySQL's "Invalid Use of Group Function" Error

Problem:

A query designed to identify parts supplied by at least two suppliers resulted in a "Invalid use of group function" error:

<code class="language-sql">SELECT c1.pid                      -- Select part ID (pid)
FROM Catalog AS c1                 -- From the Catalog table
WHERE c1.pid IN (                  -- Where pid is in the set:
    SELECT c2.pid                  -- Of pids
    FROM Catalog AS c2             -- From the Catalog table
    WHERE c2.pid = c1.pid AND COUNT(c2.sid) >= 2 -- Incorrect: COUNT in WHERE clause
);</code>

The Issue:

The error stems from the improper placement of the COUNT() function. The WHERE clause filters individual rows before grouping, making the aggregate function COUNT() invalid in this context.

Solution:

The correct approach involves using the HAVING clause, which filters after grouping and aggregation:

<code class="language-sql">SELECT c1.pid
FROM Catalog AS c1
WHERE c1.pid IN (
    SELECT c2.pid
    FROM Catalog AS c2
    GROUP BY c2.pid
    HAVING COUNT(c2.sid) >= 2
);</code>

Explanation:

The revised query groups the Catalog table by pid using GROUP BY c2.pid. The HAVING clause then filters these groups, retaining only those where the count of supplier IDs (sid) is greater than or equal to two. This correctly identifies parts with at least two suppliers. The outer query then selects the pid values from these filtered groups.

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