Quick Answer
The answer is to add GROUP BY p.ProductID, p.ProductName, p.Quantity. This fixes the SQL GROUP BY with aggregate function error because whenever you use an aggregate function like SUM in a SELECT statement, every non-aggregated column must appear in a GROUP BY clause; without it, the database treats all rows as one group, causing unpredictable results. In this case, the LEFT JOIN creates NULL values for products with no sales, and without proper grouping, the engine incorrectly displays the p.Quantity value in the TotalSold column instead of NULL. On the CompTIA ITF+ FC0-U61 exam, this tests your understanding of how aggregate functions interact with JOINs and the necessity of explicit grouping—a common trap is assuming a query that runs without errors is logically correct. Remember the mnemonic: “Every non-aggregate column must be in the GROUP BY, or your sums will lie.”
This FC0-U61 practice question tests your understanding of database fundamentals. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An inventory database contains Products (ProductID int PK, ProductName varchar(50), Quantity int, Price decimal) and Sales (SaleID int PK, ProductID int FK, SaleDate date, QuantitySold int). A report is needed: for each product, show product ID, name, current quantity, and total quantity sold. An employee writes: SELECT p.ProductID, p.ProductName, p.Quantity, SUM(s.QuantitySold) AS TotalSold FROM Products p LEFT JOIN Sales s ON p.ProductID = s.ProductID ORDER BY p.ProductID; The query executes without error but the results are incorrect. For products with no sales, TotalSold displays the same value as p.Quantity. For products with sales, TotalSold shows the correct sum. Which action should the employee take to fix the query?
Replace LEFT JOIN with INNER JOIN
Why wrong: Would exclude products with no sales.
Add 'GROUP BY p.ProductID, p.ProductName, p.Quantity'
Corrects aggregation by grouping per product.
Use SUM(COALESCE(s.QuantitySold,0))
Why wrong: Does not fix the missing GROUP BY.
Remove the ORDER BY clause
Why wrong: ORDER BY is not the cause.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Add 'GROUP BY p.ProductID, p.ProductName, p.Quantity'
The query uses an aggregate function (SUM) without a GROUP BY clause, which causes SQL to treat the non-aggregated columns (p.ProductID, p.ProductName, p.Quantity) as part of a single group. For products with no sales, the LEFT JOIN produces NULL for s.QuantitySold, and SUM(NULL) returns NULL, but the database engine incorrectly displays p.Quantity in the TotalSold column because the ungrouped query collapses rows unpredictably. Adding GROUP BY p.ProductID, p.ProductName, p.Quantity ensures SUM(s.QuantitySold) is calculated per product, returning NULL (or 0 if COALESCE is used) for products with no sales.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Replace LEFT JOIN with INNER JOIN
Why it's wrong here
Would exclude products with no sales.
Add 'GROUP BY p.ProductID, p.ProductName, p.Quantity'
Why this is correct
Corrects aggregation by grouping per product.
Related concept
Read the scenario before looking for a memorised answer.
Use SUM(COALESCE(s.QuantitySold,0))
Why it's wrong here
Does not fix the missing GROUP BY.
Remove the ORDER BY clause
Why it's wrong here
ORDER BY is not the cause.
Common exam traps
CompTIA often tests the requirement for GROUP BY when mixing aggregate and non-aggregate columns, exploiting the misconception that adding COALESCE or changing join types alone resolves the issue.
Detailed technical explanation
In SQL, when an aggregate function like SUM is used with non-aggregated columns in the SELECT list, the database engine requires a GROUP BY to define groups; without it, the query is invalid in strict SQL modes (e.g., ONLY_FULL_GROUP_BY in MySQL), but in lenient modes, it arbitrarily picks values from one row per group. The LEFT JOIN ensures products with no sales have NULL in s.QuantitySold, and SUM(NULL) yields NULL, but the ungrouped query may display the first row's p.Quantity value as TotalSold due to undefined behavior. In real-world reporting, this bug can silently produce misleading totals, making GROUP BY essential for correct aggregation.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the FC0-U61 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Database Fundamentals — This question tests Database Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: Add 'GROUP BY p.ProductID, p.ProductName, p.Quantity' — The query uses an aggregate function (SUM) without a GROUP BY clause, which causes SQL to treat the non-aggregated columns (p.ProductID, p.ProductName, p.Quantity) as part of a single group. For products with no sales, the LEFT JOIN produces NULL for s.QuantitySold, and SUM(NULL) returns NULL, but the database engine incorrectly displays p.Quantity in the TotalSold column because the ungrouped query collapses rows unpredictably. Adding GROUP BY p.ProductID, p.ProductName, p.Quantity ensures SUM(s.QuantitySold) is calculated per product, returning NULL (or 0 if COALESCE is used) for products with no sales.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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