Quick Answer
The answer is a LEFT JOIN, because it ensures all rows from the left table—in this case, the customer information dataset—are included in the result, regardless of whether a matching row exists in the right table (orders). When a customer has no orders, the columns from the orders table simply appear as NULL values, which perfectly satisfies the requirement to retain every customer record. On the CompTIA Data+ DA0-001 exam, this scenario tests your understanding of join types and their practical use in data analysis, often appearing in questions that ask you to preserve unmatched records from one table. A common trap is confusing LEFT JOIN with INNER JOIN, which would silently drop customers without orders. To remember: think of the LEFT JOIN as “left table gets all the love”—it keeps every row from the left side, even if the right side has nothing to offer.
This DA0-001 practice question tests your understanding of mining and acquiring data. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
A data analyst needs to combine two datasets: one contains customer information (customer_id, name, address) and the other contains order information (order_id, customer_id, order_date). The analyst wants to include all customers, even those who have not placed orders. Which type of join should be used?
FULL OUTER JOIN
Why wrong: FULL OUTER JOIN includes all rows from both tables, which is not necessary.
INNER JOIN
Why wrong: INNER JOIN excludes customers without orders.
LEFT JOIN
LEFT JOIN includes all customers, with order data where available.
RIGHT JOIN
Why wrong: RIGHT JOIN includes all orders, not all customers.
Answer choices
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 the 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.
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.
FULL OUTER JOIN
Why it's wrong here
FULL OUTER JOIN includes all rows from both tables, which is not necessary.
INNER JOIN
Why it's wrong here
INNER JOIN excludes customers without orders.
LEFT JOIN
Why this is correct
LEFT JOIN includes all customers, with order data where available.
Related concept
Read the scenario before looking for a memorised answer.
RIGHT JOIN
Why it's wrong here
RIGHT JOIN includes all orders, not all customers.
Common exam traps
The trap here is that candidates often confuse LEFT JOIN with INNER JOIN, assuming all customers must have orders, or they pick FULL OUTER JOIN thinking it includes all customers, but it also includes unmatched orders, which is not required.
Detailed technical explanation
In SQL, a LEFT JOIN (or LEFT OUTER JOIN) preserves all rows from the left table, using NULL placeholders for missing right-table columns. This is critical in scenarios like customer retention analysis, where you must count customers with zero orders to compute churn rates. The join condition is typically ON customers.customer_id = orders.customer_id.
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 DA0-001 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
Mining and Acquiring Data — This question tests Mining and Acquiring Data — Read the scenario before looking for a memorised answer..
The correct answer is: LEFT JOIN — A LEFT JOIN returns all rows from the left table (customers) and the 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.
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 11, 2026
This DA0-001 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 DA0-001 exam.
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