What is the unique constraint in SQL?

What is the unique constraint in SQL?

In SQL, the UNIQUE constraint is used to ensure that the values in a column or set of columns are unique across all rows in a table. This constraint prevents duplicate values from being inserted into the column or columns, and it ensures that each value in the column or columns is distinct from all other values.

Here is an example of how to use the UNIQUE constraint in SQL:

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

In this example, the UNIQUE constraint is used on the email column in the users table. This constraint ensures that no two rows in the table can have the same email address, and it prevents duplicate values from being inserted into the email column.

The UNIQUE constraint is often used in combination with the PRIMARY KEY constraint, which specifies a column or set of columns that uniquely identifies each row in the table.

In the example above, the id column is defined as the primary key for the users table, and the email column is defined as unique. This ensures that each row in the table has a unique id value and a unique email value.

Using the UNIQUE constraint can be useful when you want to ensure that the values in a column or set of columns are distinct across all rows in a table.

This constraint can help to prevent data inconsistencies and ensure the integrity of your data.

It's important to carefully consider which columns should have the UNIQUE constraint, as adding this constraint to a column can impact the performance of your database and the way you query and manipulate your data.