Candidate Key in SQL

What is SQL Candidate Key? Primary & Candidate Key.

SQL candidate key!!!

SQL stands for Structured Query Language and it is a programming language used to interact with databases. It has a very important concept of the keys that plays an important role in relational databases.

The candidate key is one of them.

Candidate keys are the most popular, and even sometimes the only, choices to be made as those that can serve as primary keys in databases.

The SQL candidate key is a column in a table that can be used to uniquely identify each row. For instance, the Social Security Number (SSN) could be a candidate key for the Employee table which contains information about employees of an organization.

In this blog, we will discuss more candidate keys in a detailed way. Furthermore, we will also differentiate between primary and candidate keys so that you can be more clear about both keys.

Let’s get started

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What is Candidate Key in SQL?

 Candidate Key in SQL

The SQL candidate key is a term used frequently in relational database writing. Most experts will advise that users pick the candidate key rather than use something else as its primary key.

A candidate key, also known as a unique key, is a field or set of fields in a table that uniquely identifies each row in the table and no two rows can share the same candidate key.

Candidate keys are also known as alternate keys and are usually chosen by the database designer before any data has been entered into the database.

Candidate keys must satisfy two conditions: They should be unique and they should not be nullable. If you try to create a candidate key on an existing table, SQL will automatically mark it as not nullable and index it when it’s created.

Difference Between Primary Key & Candidate key in SQL

As we have discussed the candidate key above It’s important to know the difference between these two keys and when to use which one!

Following are the difference between primary key and candidate key in SQL:

  1. One of the most important differences between both the keys in a table or relation is that we can have only one primary key, but there can be multiple candidate keys.
  2. The primary key is uniquely identified in a table or relation. It means it follows only one key per table practice. But the candidate key has more than one candidate and anyone of them can be selected as a primary key.
  3. The attribute which is selected as a primary key has always a unique value and doesn’t have any null value because the main functionality of a primary key is to identify the record in a relation uniquely. However, in the candidate key, It may contain null values unless attribute constraint is specified as not null.
  4. The primary key is a minimal super key which means it has one and only primary key in a relation. However, the candidate key in SQL is not a minimal super key.
  5. Each primary key can be a candidate key but a candidate key cannot be a primary key. Vice-Versa is not possible.


This article has covered in detail SQL candidate key differences with a primary key. Now you should understand what is SQL candidate key and how it is different from the primary key in SQL. If we still have any confusion, you can comment below and I will be happy to assist you.

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