At companies like Notion, interviewers often ask this to test whether you understand table design, data integrity, and constraint behavior in relational databases.
Explain the difference between a primary key and a unique key in PostgreSQL. Your answer should cover:
NULL valuesKeep 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.