Can you give examples of unique keys and explain their features?

A UNIQUE key in a database is a constraint that guarantees all values in a column or a set of columns are unique, meaning no duplicates.

Features of unique keys:

• Unlike a primary key, a unique key can contain NULL values (depending on the DBMS).
• Multiple unique keys can be created in a single table.
• Unique keys ensure data integrity by preventing duplication.

Example in SQL:

CREATE TABLE users (
    id INT PRIMARY KEY,
    email VARCHAR(255) UNIQUE,
    username VARCHAR(50),
    UNIQUE (username)
);

Here, email and username have unique constraints, which prevent inserting two records with the same email or username.