What is a unique key?

A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table. You can say that it is a little like a primary key but it can accept only one null value and it cannot have duplicate values.

Here are the steps to understand what a unique key is:

1. Understanding Keys in Databases: In databases, a key is a set of attributes that helps you identify a record (row) in a table. A table can have multiple keys.

2. Understanding Unique Key: A unique key is a type of key that is set for a column (or set of columns) in a table where all values are unique. That is, the values in this column are different for every row in the table.

3. Difference between Unique Key and Primary Key: While both primary and unique keys ensure uniqueness, the primary key makes a non-null and unique constraint on the column. On the other hand, a unique key makes a unique constraint on the column, but it can have one null value.

4. Usage of Unique Key: Unique keys are used when data in a certain column must be unique and can be used to prevent duplicate entries in that column. For example, in a table of customers, the email column might have a unique key to prevent two customers from having the same email address.

5. Creating a Unique Key: In SQL, you can create a unique key while creating a table (using CREATE TABLE command) or after creating a table (using ALTER TABLE command). The syntax is: UNIQUE(column_name).