Middle+

What are FK and UNIQUE constraints in a database?

sobes.tech AI

Answer from AI

FK (Foreign Key) — a database constraint that ensures referential integrity between tables. It guarantees that the value in a column (or set of columns) matches a primary key value in another table. This prevents the appearance of "dangling" references.

UNIQUE constraint — a constraint that guarantees all values in a column (or group of columns) are unique, i.e., no duplicates. This is useful for ensuring data uniqueness, such as a unique user email.

Example in SQL:

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  email VARCHAR(255) UNIQUE
);

CREATE TABLE orders (
  id SERIAL PRIMARY KEY,
  user_id INT,
  FOREIGN KEY (user_id) REFERENCES users(id)
);

Here, email is unique, and user_id in the orders table references an existing user in users.

Previous Next