What is the difference between a primary key and a foreign key?

Short answer: Primary Key: A unique identifier for each record in a database table. No two rows in a table can have the same primary key value. It ensures entity integrity.

Example code

user_id in a users table. Foreign Key: A field (or a combination of fields) in one table that uniquely identifies a row of another table. It establishes a relationship between two tables and enforces referential integrity. Example: user_id in an orders table linking to user_id in the users table.

Real-world example (ShopNest)

ShopNest’s SQL Server database stores customers, products, and orders. Good indexes and clear foreign keys keep checkout queries fast and safe.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.