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:

1. What each constraint does
2. Whether each allows NULL values
3. How many of each can exist on a table
4. When you would choose one over the other
5. 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.