Updated April 1, 2023
Introduction to Primary Key
The following article provides an outline for Primary Key and Foreign Key. A primary key is a non-null or unique key which identifies each record in a table or relation. Every row of a table in a database requires a unique identifier, and the primary key plays a critical role in uniquely identifying rows in the table. Duplicate values cannot be stored in the primary key column. Because we can’t provide more than one primary key in any relationship, it’s also known as a minimal super key. It is also used to uniquely identify every record in the database table. The value of the primary key column can never be NULL.
There can only be one primary key in a table. Because the primary key is a unique attribute, it cannot be used to hold duplicate values in relation. The primary key is automatically indexed because it is a clustered index by default. The table’s primary key value cannot be changed. If the user wants to delete it, make sure the referenced foreign key doesn’t have its value in it. Regardless of whether the values are contained in a foreign key or not, we can insert them into the primary key column without restriction. The primary key cannot have a parent-child connection/relation in the table. It can be defined on the temporary tables.
For example, there is a customer database with attributes like Name, ID, and City. Because each client has a unique identification number, the ID field can never include duplicate or NULL values. This feature supports the unique identification of each database record. As a result, the ID attribute can be used as a primary key.
What is Foreign Key?
A foreign key is a set of one or more columns in a database that uniquely identifies another database record in another table to ensure referential integrity. It is known as the reference key, and it’s responsible for establishing a link between two different tables in a database. A foreign key column in one table must always match the main key column in another. It means that one table’s foreign key column relates to another table’s primary key column. In relational database normalisation, a foreign key is useful, specifically when there is a need to access records from several other tables. A foreign key establishes a parent-child connection between tables, with the parent table storing the original column values and the child table referencing them. Only when the foreign key restriction is found on the child table can we achieve this relationship.
The foreign key is implemented to connect two tables. It means that one table’s foreign key relates to another table’s primary key. It accepts a NULL value. A table can contain one or more foreign keys. The foreign key column allows users to store duplicate values. It’s not a clustered index by default. The user has to create clustered index manually. The value of the foreign key can be deleted from the table without affecting the primary key of some other table. A value that does not exist in a primary key’s column cannot be added to the referenced foreign key. A foreign key constraint cannot be defined on the temporary tables. Foreign keys can create a parent-child relationship.
For example, there is a database with attributes like ID, student_Info, as well as course. The student_Id column can be made a foreign key here.
| ID | student_Id | student_Info | course |
| NULL | 1 | jon | maths |
| NULL | 4 | merry | science |
| NULL | 3 | smith | history |
| NULL | 2 | alex | history |
| NULL | 6 | rajesh | science |
| NULL | 5 | emma | maths |
We may define the foreign key in the contact table as follows to erase the referential data that eliminates records from both tables:
Code:
FOREIGN KEY (student_Id) REFERENCES student(ID)
ON DELETE CASCADE
ON UPDATE CASCADEWhen user delete a record from the student table, the relevant rows in the table are likewise deleted, and both tables are automatically updated.
Here is another example of an employee and sales table.
EMPLOYEE:
| EMP_ID | EMP_NAME | EMP_PHONE | EMP_AGE | SALES_ID |
| E01 | Ram | 9654342321 | 26 | S04 |
| E02 | Shyam | 9154342322 | 31 | S05 |
| E03 | Pooja | 9456342323 | 42 | S01 |
| E04 | Meena | 9654342301 | 24 | S02 |
| E05 | Kajal | 9874342453 | 35 | S03 |
| E06 | Rakesh | 9754342326 | 31 | S06 |
SALES:
| SALES_ID | EMP_ID | SALES_NO |
| S01 | E03 | 1 |
| S02 | E04 | 2 |
| S03 | E05 | 3 |
| S04 | E01 | 4 |
In the table, EMP_NAME, as well as EMP_ID both, are candidate keys for relation EMPLOYEE table but SALES_ID is the primary key. Only one out of many employee keys.
Conclusion – Primary Key and Foreign Key
In this article, We’ve come to the conclusion that both keys are important in relational database schemas because they build relationships between many tables. The value in the primary key column is always unique for each record in the table, however, the value in the foreign key column can be replicated. The primary key uniquely identifies a record in a table or relation, whereas the secondary key identifies a record in a table or relation. In addition, a foreign key connects two tables. So, in this article, we have seen what is the primary key and foreign key along with examples.
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