Question

Define the term:
i) Candidate key
ii) ii) Primary key

Answer 1

Of course. Here are the definitions for Candidate Key and Primary Key, explained with their properties and a clear example.

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i) Candidate Key

Definition:
A Candidate Key is an attribute (a single column) or a set of attributes (multiple columns combined) that can uniquely identify every row (or record) in a table.

A table can have multiple Candidate Keys. For an attribute or set of attributes to qualify as a Candidate Key, it must satisfy two essential properties:

1. Uniqueness: The value of the key must be unique for each row in the table. No two rows can have the same value for the Candidate Key.
2. Minimality (or Irreducibility): The key must be minimal. This means if the key is composed of multiple attributes, no attribute can be removed from it without losing the uniqueness property. For a single-attribute key, this property is automatically satisfied.

Think of Candidate Keys as "candidates" for the role of the main identifier of the table.

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ii) Primary Key

Definition:
A Primary Key is the one Candidate Key that is chosen by the database designer to be the main and official identifier for the rows in a table. It is the key used to uniquely identify each record and establish relationships with other tables.

While a table can have several Candidate Keys, it can have only one Primary Key.

In addition to the properties of a Candidate Key (Uniqueness and Minimality), a Primary Key has one extra, strict rule:

1. Cannot be NULL: A Primary Key value must exist for every record in the table. It cannot be left empty or undefined (NULL).

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Example to Illustrate Both

Let's consider a simple Students table:

| StudentID | SSN | Email | FirstName | LastName |
| :--- | :--- | :--- | :--- | :--- |
|