What Is The Difference Between A Unique Key and Primary Key In An SQL Database?

When working with SQL databases, you need to use the unique key and the primary key as fields in the database tables. However, both these keys are different from one another. The primary key refers to a single or many columns under a group present under a table. This key detects every row in the table. The Primary Key in the SQL database cannot be duplicated, and the same value cannot appear again in the table.  

Note the table in the database must have more than a single primary key that is defined at the table or column levels. If you make a primary key composite in nature, it should always be at the database’s table level.

An overview of the unique key 

The unique key refers to a group of single or over one field or column in a table that identifies the database’s record uniquely. It is like the primary key; however, it can accept a single null value for the column of the database table. The unique key cannot have the same values. It is generally referenced by the foreign key of the other tables.

What are the significant differences between the unique and primary keys?

The following are the main differences between the primary and the unique keys of the database-

1. The database can have a single primary key in the table, which has many unique keys.
2. The primary key has an objective to enforce the entity integrity, and the unique key has the unique key aims to execute the unique data.
3. In the case of the primary key, you will find that the default index in nature is clustered. The unique key, on the other hand, does not have an index that is clustered. It is non-clustered in nature. 
4. The primary key does not permit null columns, and the unique key allows null columns.
5. The duplicate keys in the case of a primary key are not permitted. In the case of the unique key, if a single or more of its vital parts remain null, the duplicate keys are permitted. 

Understanding an index further 

An index is any key that is created from a single or more database column. The index accelerates fetching the rows from the view or column. The vital aids the database like SQL Server, Oracle, MySQL servers, etc. to search for the rows associated with the fundamental values fast.  

There are two kinds of indexes. They are the-

1. Clustered index 
2. Non-clustered index

What are the primary differences between the clustered index and the non-clustered index?

The following are the primary differences between the non-clustered index and the clustered index-

1. The clustered index is a kind of index that sorts rows of data into the table based on the key values. The non-clustered index keeps the data at a single point. It stores indices at a different location.
2. The cluster index keeps the pages of the data in the leaf nodes in the index. On the other hand, the non-clustered index does not store the pages of the data in the leaf node. 
3. The cluster index does not need extra disk space. The non-clustered index needs extra disk space. 
4. The cluster index gives you quick access to data, whereas the non-cluster index here is slower.

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Reasons to use the Primary Key 

The following are the top reasons for you to deploy the primary key-

• The primary key has the sole aim to detect every record in the table of the database. 
• The primary key can be used when someone does not enter null values
• The data integrity of the database is ensured when a record is deleted or updated. 
• You should carry out the restrict operations to reject, update, or delete any action for parent tables.
• The data should be organized in the sequence of a clustered index when you need to organize the table of the database management system. 

Reasons for you to use the unique key

The following are the top reasons to use the Unique Key-

• The objective of the unique key is to ensure the data that is in the column for every table recorded is unique 
• When you have allowed the entry of a null value 
• This key is deployed when you create a non-clustered by default 
• When a single or more column or field of the table uniquely recognizes the record in the database table. 

Features of the primary key

The following are the salient features of the primary key 

1. It incorporates the table’s entity integrity 
2. The user can retain a single primary key in a table 
3. The primary key has a single or more column in the table
4. The columns are not defined as null

Here are the essential features of a unique key

1. More than a single unique key can be defined in the database table 
2. Unique keys by default are in indexes that are non-clustered in nature 
3. It has more than a single or more column in the database table
4. The column of a table might be null however only a single null column is preferred 
5. The unique key can be referenced simply by any foreign key. 

Therefore, the primary and unique keys play a crucial role when it comes to the database management system. They each have their functions and should be used correctly. When it comes to the question of which one is better, it is the unique key, especially when there are columns in the database that should not have duplication. It becomes an ideal way for you to validate data. On the other hand, the primary key is prudent when you cannot keep a null value in the database table. You can use it when there is a foreign key in another table for making the relationship. 

Jaymin Dangi