Databricks Certified Data Engineer - Associate

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Explanation:

Correct Answer: D: LEFT OUTER JOIN

Detailed Explanation:

The requirement is to get all customers (including those who have never placed an order), along with their order details when they exist.

This is a classic "all records from left table + matching records from right table" scenario.

Why LEFT OUTER JOIN?

SELECT 
    c.*,
    o.*
FROM customers c
LEFT OUTER JOIN orders o 
    ON c.customer_id = o.order_customer_id

SELECT 
    c.*,
    o.*
FROM customers c
LEFT OUTER JOIN orders o 
    ON c.customer_id = o.order_customer_id

All rows from customers are returned.
For customers with orders → order details are shown.
For customers without orders → order columns will be NULL.
This perfectly matches the requirement.

Why Other Options Are Wrong:

Option	Join Type	Result	Why Incorrect?
A	RIGHT OUTER JOIN	All orders + matching customers	Misses customers who have no orders
B	INNER JOIN	Only customers who have placed at least one order	Excludes customers with no orders
C	FULL OUTER JOIN	All customers + all orders (including orphan orders)	Returns extra rows (orders without customers) and is less efficient
E	Either LEFT or RIGHT	Technically possible if you swap table order	Misleading. While you can achieve it with RIGHT JOIN by reversing tables, the natural and expected way (given `customers` is mentioned first) is LEFT JOIN.

Note: Option E is a common trap in certification exams. While semantically equivalent if you rewrite the query, the standard and most readable solution is LEFT OUTER JOIN when you want all records from the primary (left) table.

Key Takeaway for Exam:

LEFT JOIN → "All from left, optional from right" (most common for "all customers + their orders")
RIGHT JOIN → Rarely used in practice; usually indicates the join order should be flipped
FULL JOIN → Use only when you truly need unmatched rows from both sides

This concept is heavily tested in the Databricks Data Engineer Associate exam under Lakehouse and SQL sections.

Explanation:

Correct Answer: D: LEFT OUTER JOIN

Detailed Explanation:

The requirement is to get all customers (including those who have never placed an order), along with their order details when they exist.

This is a classic "all records from left table + matching records from right table" scenario.

Why LEFT OUTER JOIN?

SELECT 
    c.*,
    o.*
FROM customers c
LEFT OUTER JOIN orders o 
    ON c.customer_id = o.order_customer_id

SELECT 
    c.*,
    o.*
FROM customers c
LEFT OUTER JOIN orders o 
    ON c.customer_id = o.order_customer_id

All rows from customers are returned.
For customers with orders → order details are shown.
For customers without orders → order columns will be NULL.
This perfectly matches the requirement.

Why Other Options Are Wrong:

Option	Join Type	Result	Why Incorrect?
A	RIGHT OUTER JOIN	All orders + matching customers	Misses customers who have no orders
B	INNER JOIN	Only customers who have placed at least one order	Excludes customers with no orders
C	FULL OUTER JOIN	All customers + all orders (including orphan orders)	Returns extra rows (orders without customers) and is less efficient
E	Either LEFT or RIGHT	Technically possible if you swap table order	Misleading. While you can achieve it with RIGHT JOIN by reversing tables, the natural and expected way (given `customers` is mentioned first) is LEFT JOIN.

Note: Option E is a common trap in certification exams. While semantically equivalent if you rewrite the query, the standard and most readable solution is LEFT OUTER JOIN when you want all records from the primary (left) table.

Key Takeaway for Exam:

LEFT JOIN → "All from left, optional from right" (most common for "all customers + their orders")
RIGHT JOIN → Rarely used in practice; usually indicates the join order should be flipped
FULL JOIN → Use only when you truly need unmatched rows from both sides

This concept is heavily tested in the Databricks Data Engineer Associate exam under Lakehouse and SQL sections.

Comments (1)

Filipe PereiraMay 20, 2026 3:27 PM

This is incorrect. The question clearly states the join condition with customers on the left. If we were supposed to consider that customers could be on the right, then why would you write in the question "join two tables, customers and orders, on the condition c.customer_id = o.order_customer_id"? The correct answer is LEFT OUTER JOIN.

LeetQuiz .May 23, 2026 7:48 AM

Hello Filipe, Thank you for your comment. You’re right, the correct choice is **LEFT OUTER JOIN** because it returns every customer (the left table) and includes order details only when they exist, leaving NULLs for customers without orders. We appreciate your feedback.

Comments (1)

Filipe PereiraMay 20, 2026 3:27 PM

LeetQuiz .May 23, 2026 7:48 AM

A data engineer needs to join two tables, `customers` and `orders`, on the condition `c.customer_id = o.order_customer_id`. The goal is to retrieve details of all customers, including those who have not placed any orders, along with the details of orders they have placed. Given the schemas of both tables, which join operation should be used to achieve this?

Real Exam

Last updated: May 23, 2026 at 07:48

RIGHT OUTER JOIN

6.8%

INNER JOIN

4.4%

FULL OUTER JOIN

15.4%

LEFT OUTER JOIN