What is a foreign key and why do we use it?
PICTURE THIS: DATABASE INDEX
Simple meaning
A foreign key is a column, or set of columns, that must match a unique key in another table.
WHY — SQL instead of guessing?
Why interviewers care about SQL:
who only read docs from people who shipped.
and tied to Backend 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:
- 1A foreign key is
a column, or set of columns, that must match a unique key in another table.
- 2It preserves referential integrity
so you cannot attach an order to a missing customer.
- 3Cascades define what happens
on parent delete or update
- 4product teams often restrict
deletes instead of cascading blindly.
- 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
A foreign key is a column, or set of columns, that must match a unique key in another table. It preserves referential integrity so you cannot attach an order to a missing customer.