SQL JOINS

Primary & Foreign Key

Just a class has a roll number for every student, a table in SQL has a primary key for every row. Foreign key is your roll number in another table which contains your fee installments. 

Primary key is a column in a table which has unique values to identify each row in that table for example your roll number is your class. Foreign key is a column in fees table which links roll number back to the class table. 

In the FEES table, you probably have a different Primary Key for the payment itself (like Receipt_ID).

The Rule: In the FEES table, the Receipt_ID is the Primary Key (each receipt is unique), but the Roll_Number is the Foreign Key (linking that receipt to a specific student).

“A Primary Key says: ‘This is me.’ A Foreign Key says: ‘This belongs to them.’“