Primary Key vs. Unique Key

Attribute	Primary Key	Unique Key
Definition	A primary key is a column or a set of columns that uniquely identifies each row in a table.	A unique key is a column or a set of columns that ensures the values in the column(s) are unique, but it allows null values.
Number of Occurrences	Only one primary key can exist per table.	Multiple unique keys can exist per table.
Null Values	A primary key cannot contain null values.	A unique key can contain null values.
Indexing	Automatically indexed by the database system for faster retrieval.	May or may not be indexed, depending on the database system.
Table Constraints	Primary key enforces both uniqueness and non-nullability constraints.	Unique key enforces only uniqueness constraint.
Relationships	Primary key is often used to establish relationships with other tables.	Unique key can also be used to establish relationships, but it is less common.

Introduction

In the world of databases, keys play a crucial role in ensuring data integrity and facilitating efficient data retrieval. Two commonly used types of keys are the Primary Key and Unique Key. While both serve the purpose of uniquely identifying records within a table, they have distinct characteristics and use cases. In this article, we will delve into the attributes of Primary Key and Unique Key, exploring their similarities and differences.

Primary Key

A Primary Key is a column or a set of columns that uniquely identifies each record in a table. It serves as the main identifier for a table and ensures that no two records have the same key value. The Primary Key constraint enforces this uniqueness and also provides a way to establish relationships between tables through foreign keys.

One of the key attributes of a Primary Key is its non-nullability. Each record in a table must have a value for the Primary Key column(s), and this value cannot be null. This ensures that the Primary Key is always present and can be used to identify a specific record.

Another important attribute of a Primary Key is its immutability. Once a Primary Key value is assigned to a record, it should not be changed. This guarantees the stability of the identifier and prevents any potential data inconsistencies that may arise from modifying the Primary Key value.

Additionally, a Primary Key is typically indexed, which means that the database engine creates an index on the Primary Key column(s). This index enhances the performance of queries that involve searching or joining based on the Primary Key.

Lastly, a table can have only one Primary Key. It uniquely identifies the records within the table and cannot be duplicated. However, a Primary Key can consist of multiple columns, forming a composite key, which allows for more complex identification scenarios.

Unique Key

A Unique Key, as the name suggests, also ensures the uniqueness of values within a column or a set of columns. It guarantees that no duplicate values are allowed in the specified column(s). However, unlike a Primary Key, a Unique Key does not have the additional constraints of non-nullability and immutability.

Similar to a Primary Key, a Unique Key can be defined on a single column or a combination of columns. This flexibility allows for more granular control over the uniqueness of records within a table.

While a Unique Key does not enforce non-nullability, it can still be used on columns that allow null values. In such cases, multiple null values are allowed, but any non-null value must be unique within the column(s) defined by the Unique Key.

Unlike a Primary Key, a table can have multiple Unique Keys. Each Unique Key provides an alternative way to identify records within the table, allowing for different unique combinations of column values.

Although a Unique Key is not automatically indexed like a Primary Key, it is often beneficial to create an index on the Unique Key column(s) to improve query performance when searching or joining based on the unique values.

Comparison

Now that we have explored the attributes of Primary Key and Unique Key, let's compare them based on various aspects:

Uniqueness

Both Primary Key and Unique Key ensure the uniqueness of values within the specified column(s). No two records can have the same key value, regardless of whether it is a Primary Key or a Unique Key.

Nullability

A Primary Key enforces non-nullability, meaning that every record in the table must have a value for the Primary Key column(s). On the other hand, a Unique Key does not impose this constraint and can be defined on columns that allow null values.

Immutability

A Primary Key value should not be changed once assigned to a record. This ensures the stability and consistency of the identifier. In contrast, a Unique Key does not have this immutability requirement, allowing the values within the Unique Key column(s) to be modified.

Indexing

While a Primary Key is automatically indexed by most database systems, a Unique Key does not have this automatic indexing. However, it is often beneficial to manually create an index on the Unique Key column(s) to improve query performance.

Number of Keys

A table can have only one Primary Key, which uniquely identifies the records within the table. On the other hand, a table can have multiple Unique Keys, each providing an alternative way to identify records based on different combinations of column values.

Conclusion

Primary Key and Unique Key are both essential components of database design, ensuring the uniqueness and integrity of data. While Primary Key serves as the main identifier for a table, enforcing non-nullability and immutability, Unique Key provides an alternative means of ensuring uniqueness without these additional constraints. Both keys can be indexed to improve query performance, and while a table can have only one Primary Key, it can have multiple Unique Keys. Understanding the attributes and differences between Primary Key and Unique Key is crucial for designing efficient and reliable database schemas.