What is the difference between a primary key and a unique key?
PICTURE THIS: DATABASE INDEX
Simple meaning
A primary key uniquely identifies a row, cannot be NULL, and a table has only one.
WHY — SQL instead of guessing?
Why interviewers care about SQL:
contrast on SQL, not two memorised paragraphs.
the developer, then one case where picking wrong hurts.
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 primary key uniquely
identifies a row, cannot be NULL, and a table has only one.
- 2A unique constraint also
forbids duplicate values but usually allows NULLs depending on the database, and you can have several unique keys.
- 3Foreign keys typically reference
a primary key or unique key.
- 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
A primary key uniquely identifies a row, cannot be NULL, and a table has only one. A unique constraint also forbids duplicate values but usually allows NULLs depending on the database, and you can have several unique keys.