Customer Orders: LEFT vs INNER JOIN

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:

1. What rows each join returns
2. How unmatched rows are handled
3. When you would choose one over the other
4. A simple example of each using tables such as customers and orders
5. 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."