A data analyst has been asked to count the number of customers in each region and has written the following query: If there is a mistake in the query, which of the following describes the mistake?

Question

Options

• AThe query is using count('). which will count all the customers in the customers table, no matter
• BThe query is missing a GROUP BY region clause.
• CThe query is using ORDER BY. which is not allowed in an aggregation.
• DThere are no mistakes in the query.
• EThe query is selecting region but region should only occur in the ORDER BY clause.

• A
  9% (4)
• B
  81% (35)
• C
  2% (1)
• D
  5% (2)
• E
  2% (1)

Explanation

When counting customers per region, SQL requires a GROUP BY region clause to partition rows into groups before applying COUNT() - without it, the database doesn't know how to separate results by region and will throw an error or return a single aggregate across all rows. Option A is wrong because COUNT(*) or COUNT(column) correctly counts rows and is not the issue here. Option C is wrong because ORDER BY is perfectly valid alongside aggregation functions. Option E is wrong because selecting region in the SELECT clause is required (and expected) when grouping by it.

Memory tip: Think of GROUP BY as the "buckets" clause - whenever you want aggregates (COUNT, SUM, AVG) broken out by a category column, that column must appear in GROUP BY. If you SELECT a non-aggregated column without GROUP BY, that's the red flag.

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