Primary Key vs Unique Key | Dataford Interview Questions

Your question is Primary Key vs Unique Key. Take a moment with it on the right.

Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).

Problem

Context

At companies like Notion, interviewers often ask this to test whether you understand table design, data integrity, and constraint behavior in relational databases.

Question

Explain the difference between a primary key and a unique key in PostgreSQL. Your answer should cover:

What each constraint does
Whether each allows NULL values
How many of each can exist on a table
When you would choose one over the other
A simple example showing both on the same table

Scope Guidance

Keep the explanation practical rather than theoretical. The interviewer is usually looking for a clear comparison, correct handling of NULL, and an understanding that both enforce uniqueness but serve different modeling purposes. A strong answer should also mention that a primary key is the main row identifier, while a unique key is commonly used for alternate candidate identifiers such as email or username.

Your question is Primary Key vs Unique Key. Take a moment with it on the right.

Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).

Context

At companies like Notion, interviewers often ask this to test whether you understand table design, data integrity, and constraint behavior in relational databases.

Question

Explain the difference between a primary key and a unique key in PostgreSQL. Your answer should cover:

What each constraint does
Whether each allows NULL values
How many of each can exist on a table
When you would choose one over the other
A simple example showing both on the same table