Could you explain what the UNIQUE constraint in SQL means and how it works?

The UNIQUE constraint in SQL guarantees that all values in a column (or a set of columns) are unique, meaning there will be no duplicate values.

This constraint is used to ensure data integrity, for example, to prevent duplicates in a table based on a specific field.

Example of creating a table with a UNIQUE constraint:

CREATE TABLE Users (
    ID INT PRIMARY KEY,
    Email VARCHAR(255) UNIQUE
);

In this example, the Email column cannot have two identical values. If you try to insert a duplicate email, the database will throw an error.

You can also create unique constraints on multiple columns to ensure that the combination of values is unique.