The Difference Between Primary Key and Unique Key
When it comes to database design, primary key and unique key are two important concepts that are often used interchangeably. However, they serve different purposes and have distinct characteristics. In this article, we will explore the difference between primary key and unique key.
Primary Key
A primary key is a column or set of columns in a table that uniquely identifies each row in the table. It must be unique for each record in the table and cannot have null values. Additionally, a table can only have one primary key. Primary keys are critical for maintaining data integrity, as they ensure that there are no duplicate records in the table.
Unique Key
A unique key, like a primary key, is used to ensure data integrity in a table. It is a column or set of columns that have unique values for each row, but unlike a primary key, it can have null values. Furthermore, a table can have multiple unique keys.
Unique keys are often used for indexing and searching data efficiently. They can be used to prevent duplicate records, but they don’t guarantee uniqueness in the same way as primary keys do.
Key Differences
The primary differences between a primary key and a unique key are:
– The primary key is used to uniquely identify each row in a table, while a unique key is used to ensure that each value in a column or set of columns is unique.
– A primary key cannot have null values and can only be defined once, while a unique key can have null values and can be defined multiple times.
– Primary keys are often used to establish relationships between tables, while unique keys are often used for indexing and searching data.
In conclusion, primary key and unique key are two important concepts in database design that serve different purposes. While they are similar in some ways, it is crucial to understand their differences to ensure data integrity and efficient querying of the database.
Table difference between primary key and unique key
| Primary Key | Unique Key |
|---|---|
| Primary key is a column or combination of columns in a table that uniquely identifies each row in that table. | Unique key is a constraint that ensures that the values in a column or combination of columns in a table are unique. |
| Primary key cannot contain null values. | Unique key can contain null values, but only one null value. |
| Each table can have only one primary key. | Each table can have multiple unique keys. |
| Primary key can be used as a foreign key in other tables. | Unique key can also be used as a foreign key in other tables. |
| Primary key is used to enforce referential integrity in a database. | Unique key is used to ensure data accuracy and consistency. |