DA0-002 Data Concepts and Environments Practice Question
A data analyst needs to combine customer data from two tables: Customers (CustomerID, Name) and Orders (OrderID, CustomerID, Amount). Only customers who have placed at least one order should be included. Which JOIN type should be used?
⚠ Common exam trap
Candidates often choose LEFT JOIN thinking it 'includes all customers' without realizing it also includes customers with no orders, which fails the explicit condition of 'only customers who have placed at least one order'.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
INNER JOIN
An INNER JOIN returns only rows where there is a match in both tables. Since the requirement is to include only customers who have placed at least one order, the INNER JOIN on CustomerID will filter out any customer without a matching order record, exactly meeting the condition.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
INNER JOIN
Why this is correct
An INNER JOIN returns only rows where CustomerID matches in both tables, excluding customers with no orders. This directly satisfies the requirement that only customers who have placed at least one order appear in the result, while unmatched customer rows are discarded.
- ✗
LEFT JOIN
Why it's wrong here
A LEFT JOIN returns every row from Customers, including those with no matching Orders, producing NULL order columns for customers who never ordered. It is tempting because it preserves the left table, which is correct when you need all customers listed regardless of whether they have placed any orders.
- ✗
FULL OUTER JOIN
Why it's wrong here
A FULL OUTER JOIN returns unmatched rows from both tables, so customers with no orders appear with NULL OrderID values, violating the requirement to include only customers who have placed at least one order. It is tempting because it preserves all rows from both sides, which is correct when you need every customer and every order, including orphans on either side.
- ✗
RIGHT JOIN
Why it's wrong here
A RIGHT JOIN returns every row from Orders plus matching Customers, so customers without orders are excluded but orders lacking a matching customer would still appear, which the requirement forbids. It is tempting because it filters to ordering customers, yet an INNER JOIN expresses that filter without admitting orphaned order rows.
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This DA0-002 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DA0-002 exam.