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Difference Between Primary Key and Foreign Key | Testbook.com

Last Updated on Jul 31, 2023

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Primary Key Vs. Foreign Key: Unravel the distinctions between Primary Key and Foreign Key
A primary key guarantees uniqueness in a specific column, whereas a foreign key establishes a connection between two tables. Discover the key differences between a primary key and a foreign key.

Defining Primary Key
A primary key is primarily concerned with ensuring the uniqueness of a table. It is either a single column or a combination of columns that uniquely identify each row in the database. This implies that it should not contain any duplicate or NULL values.

Defining Foreign Key
A foreign key is typically used to establish a relationship between two tables. Its main function is to maintain data integrity between two different instances of an entity.

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Contrasting Primary Key and Foreign Key

S.No. Primary Key Foreign Key

1 A primary key is primarily concerned with ensuring the uniqueness of a table. It guarantees the value in the specific column is unique. A foreign key is typically used to establish a relationship between two tables.

2 Only one primary key is allowed per table. A table can accommodate multiple foreign keys.

3 Primary keys do not permit null values. Foreign keys can accept multiple null values.

4 A primary key can uniquely identify a record in a database table. A foreign key is a field in one table that serves as the primary key in another table.

5 In the case of a primary key, the value cannot be removed from the parent table. In the case of a foreign key, the value can be deleted from the child table.

6 Restrictions of a primary key can be fully defined on temporary tables. Restrictions of a foreign key cannot be defined on global or local temporary tables.

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Frequently Asked Questions

What is a Primary Key in database management?

A primary key is a column or a set of columns that uniquely distinguishes every row in the database. It is unique and doesn't contain any NULL value.

What is a Foreign Key in database management?

A foreign key is used to build a relationship between the two tables. The major purpose of the foreign key is to sustain data integrity between two separate instances of an entity.

What are the main differences between Primary Key and Foreign Key?

The main differences include: a primary key focuses on uniqueness and doesn't allow NULL values, while a foreign key builds relationships between tables and can accept multiple NULL values. Also, a table allows only one primary key but can allow more than one foreign key.

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S.No.	Primary Key	Foreign Key
1	A primary key is primarily concerned with ensuring the uniqueness of a table. It guarantees the value in the specific column is unique.	A foreign key is typically used to establish a relationship between two tables.
2	Only one primary key is allowed per table.	A table can accommodate multiple foreign keys.
3	Primary keys do not permit null values.	Foreign keys can accept multiple null values.
4	A primary key can uniquely identify a record in a database table.	A foreign key is a field in one table that serves as the primary key in another table.
5	In the case of a primary key, the value cannot be removed from the parent table.	In the case of a foreign key, the value can be deleted from the child table.
6	Restrictions of a primary key can be fully defined on temporary tables.	Restrictions of a foreign key cannot be defined on global or local temporary tables.