Primary Key in DBMS vs. Unique Key in DBMS — What's the Difference?
By Tayyaba Rehman — Published on December 11, 2023
In a DBMS, a Primary Key ensures uniqueness and cannot have null values, while a Unique Key guarantees uniqueness but can contain nulls. Both serve to identify records uniquely.
Difference Between Primary Key in DBMS and Unique Key in DBMS
Table of Contents
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Key Differences
In the realm of Database Management Systems (DBMS), a Primary Key is quintessential for uniquely identifying each record in a database table. Every table can only possess one Primary Key, and this key ensures that there are no duplicate values in the column or set of columns chosen as the Primary Key. Furthermore, columns designated as a Primary Key cannot contain null values, underlining the mandatory nature of this key in record identification.
Contrarily, a Unique Key in DBMS functions to assure the uniqueness of data across the column or set of columns it is applied to. However, unlike the Primary Key, a table can have multiple Unique Keys. Another significant difference is the treatment of null values. While a Primary Key is stringent and doesn't entertain nulls, a Unique Key can incorporate null values, although the uniqueness constraint still applies.
Considering the constraints, the Primary Key in DBMS often comes with a clustered index by default, depending on the system. This means that the physical ordering of data rows in the table aligns with the Primary Key. The Unique Key, conversely, is accompanied by a non-clustered index, meaning the physical order doesn't reflect the logical order of the key.
In terms of application, developers often opt for the Primary Key in DBMS when a column or set of columns is absolutely vital for data differentiation and no room for ambiguity is permissible. The Unique Key, with its slightly relaxed constraints, is chosen when uniqueness is essential but without the non-null and singularity requirements of the Primary Key.
A noteworthy point is that while both keys, Primary and Unique, play pivotal roles in maintaining data integrity in a DBMS, they cater to slightly different needs and scenarios. Their judicious use can ensure robust and error-free database designs.
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Comparison Chart
Definition
Uniquely identifies each record.
Ensures data uniqueness across columns.
Null Values
Cannot contain null values.
Can contain null values.
Multiplicity
Only one Primary Key per table.
Multiple Unique Keys allowed in a table.
Index Type (typically)
Clustered index.
Non-clustered index.
Mandatory/Optional
Mandatory for table differentiation.
Optional but ensures uniqueness where applied.
Compare with Definitions
Primary Key in DBMS
Ensures record uniqueness with non-null constraint.
Using 'SSN' as a Primary Key ensures no two employees share the same number.
Unique Key in DBMS
Ensures data uniqueness in a DBMS column.
The 'EmailAddress' column is a Unique Key, preventing duplicate email entries.
Primary Key in DBMS
Essential for relational database integrity.
Without a Primary Key in the user table, relational operations could falter.
Unique Key in DBMS
Can have null values but retains uniqueness.
The 'OptionalID' is a Unique Key, so while it may have nulls, no two values are the same.
Primary Key in DBMS
Typically has a clustered index.
Since 'OrderID' is the Primary Key, the data is physically ordered by it.
Unique Key in DBMS
Bolsters data integrity by preventing duplicates.
With 'TransactionCode' as a Unique Key, no two transactions share the same code.
Primary Key in DBMS
Singular in nature for each table.
Though the table has multiple columns, only 'ProductCode' is the Primary Key.
Unique Key in DBMS
Multiple can exist within a single table.
The table uses both 'PassportNumber' and 'DriverLicense' as separate Unique Keys.
Primary Key in DBMS
A unique identifier for records in a DBMS table.
The 'EmployeeID' column serves as the Primary Key in the employee database.
Unique Key in DBMS
Generally accompanied by a non-clustered index.
Though 'LicensePlate' is a Unique Key, the data isn't necessarily ordered by it.
Common Curiosities
Can a table have multiple Primary Keys?
No, a table can have only one Primary Key but can have multiple Unique Keys.
What is the main purpose of a Primary Key in DBMS?
A Primary Key uniquely identifies each record in a table, ensuring no duplicates.
Do both keys support indexing?
Yes, Primary Keys typically have clustered indexes, while Unique Keys have non-clustered ones.
How does a Unique Key differ?
A Unique Key ensures column data uniqueness but can contain null values.
Can a Unique Key become a Primary Key?
Yes, if it satisfies the Primary Key's constraints, it can be designated as one.
How many null values can a Unique Key have?
A Unique Key column can have multiple null values, but other values must be unique.
Which is more restrictive, Primary Key or Unique Key?
Primary Key, as it can't have null values and is singular per table.
Are both keys essential for normalization?
While not the sole criteria, they are crucial for achieving higher normalization forms.
How does a DBMS enforce uniqueness?
Through the constraints of Primary and Unique Keys, which prevent duplicate entries.
Why can't a Primary Key have null values?
Null denotes unknown; hence, a Primary Key with null would compromise record uniqueness.
Is a Primary Key always mandatory?
While not always mandatory, it's essential for ensuring data integrity and relations.
Can both keys span multiple columns?
Yes, both Primary and Unique Keys can be composite, spanning multiple columns.
Do all tables require a Primary Key?
Not necessarily, but it's recommended for relational integrity.
Why use a Unique Key instead of a Primary Key?
Unique Key is used when uniqueness is desired without the non-null constraint of Primary Key.
Can keys be altered after table creation?
Yes, but care must be taken to ensure data integrity isn't compromised.
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Written by
Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.
