Difference Between Primary Key And Unique Key Explained (+Examples)

9 mins read

We all know that a relational database refers to the collection of data organized in rows and columns, where there exist predefined relations between these rows and columns. But how do we define these relations? We use 'Keys' to define these relations based on different attributes.

Multiple keys make it possible to access information from large databases as and when needed. In this article, we will look at the difference between primary key and unique key. Let's read on!

What Is Primary Key?

A primary key is a key in a relational database that the Database Administrator selects to identify a tuple or row in the database table uniquely. It can then be used to create a relationship among tables i.e., the data in a particular table row can be related to data in some other table through these keys and be retrieved as and when needed. 

What Is a Unique Key?

Unique key is very similar to the primary key, barring the fact that the primary key doesn't allow null values in the column, but the unique key does. So, unique key can be defined as a unique identifier for rows in a database table that doesn't allow duplicate values and can uniquely identify a row/tuple in the database table. We can make unique key from one or more table fields.

Difference Between Primary Key and Unique Key

The fact that main purpose of both primary keys and unique keys is to act as the unique identifier for records in rows. But it may seem that the two keys are similar. However, this is not the case. To help clear the confusion surrounding this topic, the table below gives the main difference between unique and primary keys. Have a look:

This gives a clear view of the difference between unique key and primary key. Now let's move on to discussing a few other important things about these keys to get a better idea of when to use which of these keys.

Features of Primary Key

• It doesn't allow duрliсаte vаlues.
• It can be made from one or more columns of the table.
• It doesn't allow a NULL value.
• Only a single primary key per table is allowed.
• It imрlements the entity integrity оf the tаble.

Why use Primary Key?

The main purpose of using the primary key is to recognize the uniqueness of data in a row and ensure that there are no null values. This key constraint can then be used to retrieve the data for that unique attribute and also create relationships between the different tables in the database record. Take for example, the table below:

Table Name Employee

In this table, Emp_Id can be a perfect primary key for the table because Emp_Id is always unique and can never be null. Therefore, in the above table, the Emp_id is the primary key table column for our employee database record.

Features of Unique key

• Unique Keys can be made from one or more columns.
• Multiple Unique keys per table are allowed.
• It is in non-clustered unique indexes by default.
• It allows NULL value, but only one NULL is allowed per column

Why use Unique Key?

We generally use the unique key constraint to uniquely identify a tuple/ row in a table while also making room for a null value to exist. There can be multiple unique keys in a single table, and they can also be used as foreign keys for another table.

Going back to the Employee Table as mentioned in the primary key example:

In this table, Emp_Email will be unique, and it may also contain Null values for employees who don't have an email ID. So, we may enforce unique integrity constraints on the Emp_Email column, and it becomes the unique key column of the table.

Primary key vs. Unique key: Which of the two is better?

Now that we know the difference between primary key and unique key, let's see which of these is better. 

• It is more appropriate to use the unique key when you want to identify the uniqueness of data in a column of a table but also allow for null values. A unique key in the table is a good way to validate the data in this case.
• It is ideal to use a primary key when you don't want to allow for any null values in the row/ column and also want the row attribute to be unique. This is a good way to ensure uniqueness and avoid duplication. The primary values/ keys also act as the foreign key for another table in the database to create relations.

In layman's terms, a primary key is like a social security number - it's absolutely unique and identifies one specific record. A unique key is like a nickname - it should be unique within reason, but it can allow some exceptions.

Conclusion

Even though it may seem like these keys are the same at first look, now you know about the difference between primary key and unique key. The most important feature to remember is that the primary key doesn't allow a null value while the unique key constraint allows a null value. The primary key constraint is clustered unique index by default while the unique key is a non-clustered index by default. So, a unique key constraint is useful when we don't want columns to have duplicate values, and a primary key constraint is useful when we want columns neither to have duplicate values nor null values.

Frequently Asked Questions

1. What are the primary key features?

The primary key features are:

• Each row in a table is identified by a primary key, which is a unique feature.
• It can be based on one or more columns.
• For each row of data, a primary key must have a distinct value.
• It cannot contain null values.
• It enforces the entity integrity of the table.
• It uniquely identifies a row.
• It results in clustered unique indexes by default.

2. What are the main differences between primary key and unique key?

The major differences between primary key and unique key are:

3. Is a primary key column allowed to contain NULL values?

The main function of a primary key is to guarantee data integrity and give a way to specifically identify each record in a table by acting as a unique identifier for each row. The primary key column must contain only distinct, non-null values.

4. What is the syntax for the creation of a primary key?

The syntax for the creation of the primary key is:

CREATE TABLE table_name(
column1 datatype,
column2 datatype,
...,
PRIMARY KEY(column_list)
);

5. Can you change the values in a primary key column once they have been set?

It is not possible to change the values in a primary key column once they have been set. The primary key column is designed to have unique values, and changing the values would violate this constraint. If you need to change the values in a primary key column, you would need to delete the existing rows and insert new rows with the updated values. However, this approach can cause issues with foreign key constraints that reference the primary key column.

6. Why is a unique key necessary in a table?

A unique key is necessary in a table because of the following reasons:

• Enforcing data integrity: A unique key ensures that the values in a column or group of columns are unique and prevents the insertion of duplicate values into the table.
• Improving performance: A unique key can be used as an index to speed up the process of searching for duplicates.
• Supporting relationships: A relation between two tables can be created by using a unique key as a foreign key in another table.

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