What is primary and foreign key in database? Clarify it with examples.

Keys are fields that uniquely identify records and link tables together. Without keys, a database cannot maintain accuracy or relationships.

Primary Key: A primary key is a field (or combination) that uniquely identifies each record in a table. It cannot be empty (NULL) and cannot repeat (UNIQUE).

Example — Student table:

StudentID (PK)	Name	Class
101	Sita Sharma	10
102	Ram Thapa	10

Here StudentID is primary key. No two students can have 101, and it can never be blank. Phone number is a bad primary key because it can change; ID is good because it is stable.

Foreign Key: A foreign key is a field in one (child) table that refers to the primary key of another (parent) table. It creates the relationship and enforces referential integrity.

Example — LibraryIssue table:

IssueID	StudentID (FK)	Book
1	101	DBMS Book
2	102	Networking Book

Here StudentID in LibraryIssue is a foreign key pointing to Student.StudentID. You cannot issue a book to StudentID 999 if 999 does not exist in Student table — Access/MySQL will reject it.

Together: Primary key = identity of its own table; Foreign key = link to another table’s identity. Example line for exam: Department.DeptID (PK) → Employee.DeptID (FK) makes one-to-many relationship.