What is a Primary key vs Foreign key

Let’s break it down clearly:

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Primary Key (PK):

• Definition: A primary key is a column (or a set of columns) in a table that uniquely identifies each row.
• Characteristics:
  1. Must be unique for every row.
  2. Cannot contain NULL values.
  3. A table can have only one primary key.
• Example:

student_id	name	age
1	Alice	20
2	Bob	21

Here, student_id is a primary key because it uniquely identifies each student.

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Foreign Key (FK):

• Definition: A foreign key is a column (or set of columns) in a table that references the primary key in another table. It is used to maintain a relationship between two tables.
• Characteristics:
  1. Can have duplicate values.
  2. Can have NULL values (unless specified otherwise).
  3. Enforces referential integrity between tables.
• Example:

Table: Courses

course_id	course_name
101	Math
102	Physics

Table: Enrollments

enrollment_id	student_id	course_id
1	1	101
2	2	102

Here:

• course_id in Enrollments is a foreign key referencing course_id in Courses.
• student_id in Enrollments could also be a foreign key referencing student_id in Students.

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✅ Key difference:

• Primary key: uniquely identifies a row in its own table.
• Foreign key: creates a link to a primary key in another table.