8.1 The Importance of Keys

In a database, we must be able to distinguish one entity from another. Keys are attributes that allow us to uniquely identify a record.

8.2 Types of Keys

1. Super Key

A set of one or more attributes that, taken collectively, allow us to identify uniquely an entity in the entity set.

• Example: For a Student, {Roll_No}, {Roll_No, Name}, and {Email, Phone} are all Super Keys.

2. Candidate Key

A "Minimal" Super Key. A super key for which no proper subset is also a super key.

• Example: If {Roll_No} is enough to identify a student, then {Roll_No, Name} is a super key but not a candidate key (because {Roll_No} is a subset).
• A table can have multiple Candidate Keys (e.g., Roll_No and Email).

3. Primary Key

One of the Candidate Keys chosen by the database designer to be the principal means of identifying entities.

• Rules for Primary Key:

1. Must be unique.
2. Cannot be NULL.
3. Should be stable (values shouldn't change often).

• Notation: Underlined in ER diagrams.

4. Composite Key

A primary key that consists of more than one attribute.

• Example: In an Enrollment table, the combination of {Student_ID, Course_ID} might be the key.

5. Foreign Key

An attribute in one table that refers to the Primary Key of another table. It is used to create a link between two tables.