DA0-002 Data Concepts and Environments Practice Question
A data analyst needs to combine data from two tables: one containing customer information and another containing order details. The analyst wants to include all customers, even those who have not placed any orders. Which type of join should be used?
⚠ Common exam trap
A common mix-up: candidates confuse LEFT JOIN with FULL OUTER JOIN, thinking they need to preserve all rows from both tables, when the requirement only specifies preserving all customers.
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
✓
LEFT JOIN
A LEFT JOIN returns all rows from the left table (customers) and matching rows from the right table (orders). If a customer has no orders, the order columns will contain NULLs. This satisfies the requirement to include all customers, even those without orders.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
FULL OUTER JOIN
Why it's wrong here
A FULL OUTER JOIN returns all rows from both tables, matching where possible and filling unmatched rows with NULLs, which would include customers without orders but also introduces extraneous rows for orders without matching customers. The requirement explicitly demands only all customers, not all orders, so a LEFT JOIN (retaining all rows from the customer table) is the precise mechanism needed. This option is tempting because FULL OUTER JOIN is the standard choice when the goal is to preserve unmatched rows from both sides, such as when comparing complete datasets from two systems.
- ✓
LEFT JOIN
Why this is correct
A LEFT JOIN returns every row from the left (customer) table plus matching rows from the right (order) table, emitting NULLs where no order exists. That satisfies the stated constraint of retaining customers with zero orders, which an INNER JOIN would silently discard.
- ✗
INNER JOIN
Why it's wrong here
An INNER JOIN returns only rows where customer and order keys match, so customers with no orders are excluded entirely. It is tempting because it is the default join for combining related tables, and would be correct if the analyst wanted only customers who have actually placed orders.
- ✗
RIGHT JOIN
Why it's wrong here
A RIGHT JOIN preserves every row from the orders table, so customers without orders are dropped, contradicting the requirement to retain all customers. It is tempting because it also returns unmatched rows, but it does so on the wrong side; a LEFT JOIN with customers as the left table is needed here.
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Written by Johnson Ajibi, MSc IT Security
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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.