Customer Orders: LEFT vs INNER JOIN | Dataford Interview Questions

Welcome to your interview.

The question is on your right: Customer Orders: LEFT vs INNER JOIN. Take a moment with it first.

Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.

Problem

Context

Joins are one of the most common SQL interview topics because they directly affect which rows appear in a result set. A wrong join choice can silently exclude important records or include unexpected NULL values.

Core Question

Explain the difference between INNER JOIN and LEFT JOIN in SQL. In your answer, cover:

What rows each join returns
How unmatched rows are handled
When you would choose one over the other
A simple example of each using tables such as customers and orders
A common mistake people make when filtering after a LEFT JOIN

Scope Guidance

The interviewer is looking for a practical explanation, not just definitions. You should be able to describe the result set behavior clearly, explain how NULLs appear in a LEFT JOIN, and connect each join type to realistic business questions such as "customers with orders" versus "all customers, including those with no orders."

Welcome to your interview.

The question is on your right: Customer Orders: LEFT vs INNER JOIN. Take a moment with it first.

Context

Core Question

Explain the difference between INNER JOIN and LEFT JOIN in SQL. In your answer, cover:

What rows each join returns
How unmatched rows are handled
When you would choose one over the other
A simple example of each using tables such as customers and orders
A common mistake people make when filtering after a LEFT JOIN