Difference between Primary Key and Foreign Key
Last Updated : 27 Dec, 2024
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.
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 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, Primary Keys determine a row in the table and Foreign Key determines the relation between tables.