Primary Key vs Foreign Key in SQL

Primary keys and foreign keys are essential for maintaining data integrity and relationships between tables in a relational database.

🔹 Primary Key

• Uniquely identifies each row in a table.
• Cannot contain NULL values.
• Only one primary key per table (can be composite with multiple columns).

CREATE TABLE Students (
    student_id INT PRIMARY KEY,
    name VARCHAR(50)
);

🔹 Foreign Key

• Creates a relationship between two tables.
• Refers to the primary key of another table.
• Can have multiple foreign keys in one table.

CREATE TABLE Enrollments (
    enrollment_id INT PRIMARY KEY,
    student_id INT,
    course_id INT,
    FOREIGN KEY (student_id) REFERENCES Students(student_id)
);

🔹 Key Differences

Feature	Primary Key	Foreign Key
Purpose	Unique identity	Relationship between tables
NULL Allowed?	❌ No	✅ Yes
Uniqueness	Must be unique	Can repeat
Count per Table	Only 1	Many allowed