Which of the following SQL statements would be most useful for an auditor attempting to identify duplicate invoice numbers in a table named 'Invoices'?

Answer options:

SELECT DISTINCT InvoiceNum FROM Invoices

SELECT * FROM Invoices ORDER BY InvoiceNum

SELECT InvoiceNum, COUNT() FROM Invoices GROUP BY InvoiceNum HAVING COUNT() > 1

SELECT InvoiceNum FROM Invoices WHERE InvoiceNum IS NOT NULL

How to approach this question

To find duplicates, you need to count occurrences of each item and look for counts > 1. This requires GROUP BY and HAVING.

Full Answer

C.SELECT InvoiceNum, COUNT(*) FROM Invoices GROUP BY InvoiceNum HAVING COUNT(*) > 1✓ Correct

SELECT InvoiceNum, COUNT(*) FROM Invoices GROUP BY InvoiceNum HAVING COUNT(*) > 1

The GROUP BY clause groups rows with the same values, and HAVING COUNT(*) > 1 filters those groups to show only the ones that appear multiple times (duplicates).

Common mistakes

Using DISTINCT (which removes duplicates) instead of finding them.

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