Difference between candidate key & primary key

Question

What is the differences between candidate key and primary key?

Answer 1

I think this post does a pretty good job of explaining the difference.

To summarize:

Candidate Key – A Candidate Key can be any column or a combination of columns that can qualify as unique key in database. There can be multiple Candidate Keys in one table. Each Candidate Key can qualify as Primary Key.

Primary Key – A Primary Key is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key.

A table can have multiple Candidate Keys that are unique as single column or combined multiple columns to the table. They are all candidates for Primary Key.

Answer 2

Candidate keys can take null values whereas primary keys can never be null. That is also one of the significant differences.

Answer 3

Candidate Key – A Candidate Key can be any column or a combination of columns that can qualify as a unique key in the database. There can be multiple Candidate Keys in one table. Each Candidate Key can qualify as a Primary Key.

Primary Key – A Primary Key is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key.

Candidate keys can take null values whereas primary keys can never be null. That is also one of the significant differences.