Keys in Database Design with Examples:

• Candidate Key: Minimal set uniquely identifies tuple. Example: Student {rollno}, {email} - both candidates.
• Primary Key: Chosen candidate key to uniquely identify row, cannot be NULL. Example: rollno PK for Student.
• Alternate Key: Candidate keys not chosen as primary. Example: email alternate.
• Super Key: Any superset of candidate key. Example: {rollno, name}, {rollno, email} - super keys.
• Foreign Key: Attribute in one table referencing PK of another for referential integrity. Example: Enroll(s_id) references Student(rollno).
• Composite Key: Key with multiple attributes. Example: Enroll primary key (s_id, c_id).
• Unique Key: Ensures uniqueness but allows one NULL (varies by DBMS).
• Natural vs Surrogate: Natural uses real data (rollno), Surrogate artificial (auto-increment id).

Importance: ensure entity integrity, referential integrity, normalization, indexing.