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.
- ✗
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.
- ✗
RIGHT JOIN
Why it's wrong here
RIGHT JOIN includes all orders, not all customers.
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