Technical question · SQL
Explain the difference between an inner join and a left join, as if you were talking to a manager who doesn’t use SQL.
What they’re really asking
Two things are being tested at once: whether you really understand joins, and whether you can explain a technical idea in plain English. The interviewer wants one clear example, the practical difference in which rows you keep, and why a manager should care. Jargon without explanation misses the point of the question.
How to structure your answer
- Start with the purposeA join combines two tables that share something, such as a customer number.
- Use one everyday exampleTwo lists the manager already knows, so the difference is easy to picture.
- Say what each join keepsInner: only the matches. Left: everything from the first list, with gaps where there’s no match.
- Explain why it mattersThe choice changes the answer, so tie it to a question the manager might ask.
For more on answering this kind of question, including the mistakes to avoid, read our guide to technical questions.
An example outline
This is an example outline, not a script. Use it to see the shape of a strong answer, then build yours from your own experience, in your own words.
- Purpose
- We often combine two lists, such as our list of shops and this month’s sales.
- Inner join
- Only shops that appear in both lists: shops that sold something this month.
- Left join
- Every shop on the first list, with its sales where there are some and a blank where there aren’t.
- Why it matters
- To find shops that sold nothing, you need the left join, because the inner join quietly drops them.
- Check
- So if a report’s shop count looks low, the first thing I’d check is which join it used.
Try it in code
Technical interviews often ask you to write it as well as explain it. These challenges run in your browser, on realistic data, with hidden tests.
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Read the guide
- How to explain your SQL out loud in an interviewInterview guide · 7 min read
- How to talk through a data project in an interviewInterview guide · 7 min read
- How to answer a data pipeline design question as a junior data engineerInterview guide · 7 min read
Last reviewed 28 September 2026 by the Deeplink Interview team. How we write and check our questions.