Difference Between Primary Key and Foreign Key

comparison between Primary Key and Foreign Key:

Feature	Primary Key	Foreign Key
Definition	Uniquely identifies a record in a table	Establishes a relationship between two tables
Uniqueness	Must be unique for each row	Can have duplicate values
NULL Values	Cannot be NULL	Can have NULL values
Number of Keys in a Table	Only one primary key per table	A table can have multiple foreign keys
Relation	Defines the main identifier for a record	Refers to the primary key of another table
Dependency	Independent, does not depend on any other table	Always refers to a primary key in another table
Indexing	Automatically indexed	Not automatically indexed
Modification	Cannot be modified once assigned	Can be updated, but should match a valid primary key
Example	In Employees table: Emp_ID (Primary Key)	In Salaries table: Emp_ID (Foreign Key referencing Employees.Emp_ID)

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Example in SQL

1. Primary Key Example (Employees Table)

CREATE TABLE Employees (
    Emp_ID INT PRIMARY KEY,
    Name VARCHAR(50)
);

• Emp_ID is the Primary Key, ensuring each employee has a unique identifier.

2. Foreign Key Example (Salaries Table)

CREATE TABLE Salaries (
    Salary_ID INT PRIMARY KEY,
    Emp_ID INT,
    Amount DECIMAL(10,2),
    FOREIGN KEY (Emp_ID) REFERENCES Employees(Emp_ID)
);

• Emp_ID in Salaries is a Foreign Key linking to Emp_ID in Employees, ensuring data consistency.