What is a LEFT JOIN?
PICTURE THIS: DATABASE INDEX
Simple meaning
It keeps all left rows and matching right rows, with nulls when no match.
WHY — SQL instead of guessing?
Why interviewers care about SQL:
who only read docs from people who shipped.
and tied to Data Science work.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1It keeps all left
rows and matching right rows, with nulls when no match.
- 2Why it exists
Inner join drops non-matches.
- 3How it works
I sketch tables in interviews.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
It keeps all left rows and matching right rows, with nulls when no match. Inner join drops non-matches.