DA0-002 Data Concepts and Environments Practice Question
A data analyst needs to combine customer data from two tables: Customers (CustomerID, Name) and Orders (OrderID, CustomerID, Amount). Only customers who have placed at least one order should be included. Which JOIN type should be used?
⚠ Common exam trap
Candidates often choose LEFT JOIN thinking it 'includes all customers' without realizing it also includes customers with no orders, which fails the explicit condition of 'only customers who have placed at least one order'.
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
✓
INNER JOIN
An INNER JOIN returns only rows where there is a match in both tables. Since the requirement is to include only customers who have placed at least one order, the INNER JOIN on CustomerID will filter out any customer without a matching order record, exactly meeting the condition.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
INNER JOIN
Why this is correct
INNER JOIN returns only matching rows, which are customers with orders.
- ✗
LEFT JOIN
Why it's wrong here
LEFT JOIN returns all customers, even those without orders.
- ✗
FULL OUTER JOIN
Why it's wrong here
FULL OUTER JOIN returns all rows from both tables, including non-matching.
- ✗
RIGHT JOIN
Why it's wrong here
RIGHT JOIN returns all orders, but we need only customers with orders.
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