What is the candidate key in SQL Server

Question

Is it only a theoretical idea or a practical one? What distinguishes it from the primary key?

Answer 1

A Candidate Key is any column or set of columns that can be used as a database's unique key. In a single table, there might be numerous Candidate Keys. Each Candidate Key has the potential to be a Primary Key.

A primary key is a column or a set of columns that uniquely identifies a record in a database. There can only be one Candidate Key that is the primary key.

When choosing the Primary Key, one must be extremely cautious, since making the wrong choice might have a negative influence on the database architect and future normalisation. To qualify as a Primary Key, a Candidate Key must be non-NULL and unique across all domains.

Example: 

create table customers (
   customer_id int not null primary key,
   firstname varchar(50) not null,
   lastname varchar(50) not null,
   gender char(1),
   constraint uk_customers unique (customer_id)
);