DA0-002 Data Acquisition and Preparation Practice Question
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?
⚠ Common exam trap
A common mix-up: candidates 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.
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 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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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FULL OUTER JOIN
Why it's wrong here
FULL OUTER JOIN retains unmatched rows from both tables, so it also includes orders whose customer_id matches no customer. That exceeds the requirement to keep all customers. FULL OUTER JOIN is correct when unmatched records from both datasets must be preserved.
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INNER JOIN
Why it's wrong here
INNER JOIN returns only rows where customer_id matches in both tables, so customers without orders are excluded entirely. A LEFT JOIN keeping all customer rows is required. INNER JOIN is correct when the goal is only matched pairs, such as reporting solely on customers who have ordered.
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LEFT JOIN
Why this is correct
A LEFT JOIN returns every row from the customer table plus matching order rows, emitting NULLs where no orders exist. This satisfies the stem's requirement to retain customers without orders, unlike an INNER JOIN which would exclude them.
- ✗
RIGHT JOIN
Why it's wrong here
RIGHT JOIN preserves all rows from the orders table, not the customers table, so customers lacking orders still disappear. The customers table must be the preserved side via LEFT JOIN. RIGHT JOIN is correct when the second table's unmatched rows must be retained instead.
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