DA0-002 Data Concepts and Environments Practice Question
A data analyst needs to combine data from two tables: one containing customer information and another containing order details. The analyst wants to include all customers, even those who have not placed any orders. Which type of join should be used?
⚠ Common exam trap
A common mix-up: candidates confuse LEFT JOIN with FULL OUTER JOIN, thinking they need to preserve all rows from both tables, when the requirement only specifies preserving all customers.
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 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
A FULL OUTER JOIN returns all rows from both tables, matching where possible and filling unmatched rows with NULLs, which would include customers without orders but also introduces extraneous rows for orders without matching customers. The requirement explicitly demands only all customers, not all orders, so a LEFT JOIN (retaining all rows from the customer table) is the precise mechanism needed. This option is tempting because FULL OUTER JOIN is the standard choice when the goal is to preserve unmatched rows from both sides, such as when comparing complete datasets from two systems.
- ✓
LEFT JOIN
Why this is correct
LEFT JOIN includes all rows from the left table, even if no match exists in the right table.
- ✗
INNER JOIN
Why it's wrong here
INNER JOIN returns only rows with matching values in both tables, excluding customers without orders.
- ✗
RIGHT JOIN
Why it's wrong here
RIGHT JOIN includes all rows from the right table, which would not achieve the goal.
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