Difference between Primary Key and Foreign Key

Pre-Requisite: Relational Database Model

Keys are one of the most important elements in a relational database to maintain the relationship between the tables and it also helps in uniquely identifying the data from a table. The primary Key is a key that helps in uniquely identifying the tuple of the database whereas the Foreign Key is a key that is used to identify the relationship between the tables through the primary key of one table that is the primary key one table acts as a foreign key to another table. Now, let’s discuss both of them in some detail.

What is Primary Key?

A primary key is used to ensure that data in the specific column is unique. A column cannot have NULL values. It is either an existing table column or a column that is specifically generated by the database according to a defined sequence. 

Example: STUD_NO, as well as STUD_PHONE both, are candidate keys for relation STUDENT but STUD_NO can be chosen as the primary key (only one out of many candidate keys). 

Table STUDENT

STUD_NO	STUD_NAME	STUD_PHONE	STUD_STATE	STUD_COUNT	STUD_AGE
1	RAM	9865278251	Haryana	India	20
2	RAM	9655470231	Punjab	India	19
3	SUJIT	7514290359	Rajasthan	India	18
4	SURESH	8564103258	Punjab	India	21

Table STUDENT_COURSE

STUD_NO	COURSE_NO	COURSE_NAME
1	C1	DBMS
2	C2	Computer Networks
1	C2	Computer Networks

What is Foreign Key?

A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It is a column (or columns) that references a column (most often the primary key) of another table. 

Example: STUD_NO in STUDENT_COURSE is a foreign key to STUD_NO in STUDENT relation. 

Difference between Primary Key and Foreign Key

PRIMARY KEY	FOREIGN KEY
A primary key is used to ensure data in the specific column is unique.	A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables.
It uniquely identifies a record in the relational database table.	It refers to the field in a table which is the primary key of another table.
Only one primary key is allowed in a table.	Whereas more than one foreign key is allowed in a table.
It is a combination of UNIQUE and Not Null constraints.	It can contain duplicate values and a table in a relational database.
It does not allow NULL values.	It can also contain NULL values.
Its value cannot be deleted from the parent table.	Its value can be deleted from the child table.
It constraint can be implicitly defined on the temporary tables.	It constraint cannot be defined on the local or global temporary tables.

Conclusion

In this article, we have basically mentioned the primary key and foreign key, and the differences between them. Both the keys, whether the primary key or the foreign key, play an important role in the Database management system. Primary Key contains unique values, whereas Foreign Key contains values taking reference from Primary Keys. The main characteristic property of the Primary key is that it can’t be repeated, it is unique. There are some differences between their functions, as Primary Keys determines a row in the table and Foreign Key determines the relation between tables.

FAQ

1. How many types of keys are there in the Relational Database Model?

• Candidate Key
• Primary Key
• Super Key
• Alternate Key
• Foreign Key
• Composite Key

For more, you can refer to Types of Keys in the Relational Model.

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