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

Resolving MySQL "Invalid grouping function usage" error

In MySQL, when encountering the "Invalid grouping function use" error message, it is crucial to understand the difference between the WHERE and HAVING clauses.

Question:

Your query retrieves part IDs (pids) provided by at least two suppliers. However, you receive the following error:

<code>1111 - Invalid use of group function</code>

Reason:

This error results from incorrect use of the WHERE clause in the subquery. You are trying to filter rows using a WHERE clause before grouping and aggregating them, which is not expected behavior.

Solution:

The solution is to replace the WHERE clause with a HAVING clause. HAVING runs after MySQL has grouped rows and calculated aggregates, allowing you to filter results based on these aggregates.

Rewrite the subquery:

Replace the WHERE clause in the subquery with the HAVING clause:

<code>(                  -- pid属于以下集合：
SELECT c2.pid                  -- pids集合
FROM Catalog AS c2             -- 来自catalog表
WHERE c2.pid = c1.pid
HAVING COUNT(c2.sid) >= 2)</code>

Instructions:

This modified subquery will correctly filter the rows after grouping by pid and calculating the distinct supplier ID (sid) associated with each pid. The HAVING clause ensures that only rows with a count of at least 2 are selected.

By using the HAVING clause appropriately, you can resolve the "Invalid grouping function usage" error and get the results you want.

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