Databases: Primary Keys

Objective

Understand the purpose of a primary key and identify one.

What is a Database?

A database is an organized collection of data, typically stored and accessed electronically. Databases are used to store information in a structured way, making it easy to retrieve, update, and manage.

What is a Primary Key?

A primary key is a special field (or set of fields) in a database table that uniquely identifies each record (row) in that table. It's the most important field for ensuring data integrity.

Key characteristics of a primary key:

• Uniqueness: No two records can have the same primary key value.
• Not Null: A primary key cannot contain a missing value (NULL).
• Immutable: The value of a primary key should not be changed once it's assigned to a record.

Why are Primary Keys Important?

Primary keys serve several crucial purposes:

• Data Integrity: They ensure that each record is uniquely identifiable, preventing data duplication and inconsistencies.
• Efficient Data Retrieval: Databases can quickly locate specific records using their primary key.
• Relationships between Tables: Primary keys are often used to link records in one table to records in another table (foreign keys).

Examples of Primary Keys

Here are some examples of what can be used as primary keys:

• Student ID: A unique identifier for each student in a student database.
• Product Code: A unique code for each product in a product inventory.
• Account Number: A unique number for each bank account.
• Employee ID: A unique identifier for each employee in an employee database.

Identifying a Primary Key

To identify a primary key, consider the following:

1. Does the field uniquely identify each record?
2. Is the field guaranteed to have a value for every record?
3. Is the field unlikely to change over time?

Example Table

Consider a table called 'Students'.

Student ID	Name	Grade
1001	Alice Smith	10
1002	Bob Johnson	11
1003	Charlie Brown	10

In this example, 'Student ID' is a good candidate for a primary key because it is unique for each student and is unlikely to change.

Foreign Keys

A foreign key is a field in one table that refers to the primary key of another table. It establishes a link between the two tables.

Summary

A primary key is a crucial element of a well-designed database. It ensures data integrity, enables efficient data retrieval, and facilitates relationships between tables.