Difference between Primary Key and Foreign Key

Last Updated : 29 Aug, 2025

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 uniquely identify the tuple of the database. In contrast, the Foreign Key is a key used to determine the relationship between the tables through the primary key of one table that is the primary key of 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, 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.