DA0-002 Data Acquisition and Preparation Practice Question
A marketing analyst must combine two datasets: a CRM extract with one row per customer and a transactions extract with many rows per customer. The analyst wants every customer from the CRM to appear in the output, even customers with no matching transactions. Which join type should be used?
⚠ Common exam trap
The trap here is defaulting to an inner join out of habit and silently dropping the non-purchasing customers the analyst explicitly needs.
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 anchors the result on the CRM customer list and enriches it with transaction data, retaining customers who never purchased. Unmatched customers receive NULL transaction fields, which is exactly the desired behavior for a master-list enrichment. Inner, full outer, and cross joins either drop customers, add unwanted orphans, or fabricate pairings.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
LEFT JOIN
Why this is correct
A left join keeps all rows from the left table (the CRM customers) and attaches matching transaction rows where they exist, producing NULLs for customers without transactions. This satisfies the requirement that every customer appears while still enriching those who have purchase activity. It is the standard pattern for preserving a master list during an enrichment join.
- ✗
CROSS JOIN
Why it's wrong here
A cross join produces the Cartesian product of both tables, pairing every customer with every transaction regardless of the key. That massively multiplies row counts and assigns unrelated purchases to customers, which is factually wrong. It does not use a matching key at all, so it cannot satisfy the requirement to keep customers while attaching their real transactions.
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FULL OUTER JOIN
Why it's wrong here
A full outer join preserves unmatched rows from both sides, so it would also include transactions whose customer key is missing from the CRM. Those orphan transaction rows are not requested and would inflate the customer population with records that cannot be attributed to a known customer. It returns more than the scenario asks for and complicates downstream counting.
- ✗
INNER JOIN
Why it's wrong here
An inner join returns only rows where the key exists in both datasets, so customers with no transactions would drop out entirely. That directly contradicts the requirement that every CRM customer appears in the result. Inner joins are the default in many tools, which makes this an easy but incorrect choice when unmatched records must be preserved from one side.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
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.