Primary Keys in Database Fundamentals

What is a Primary Key?

A primary key is a column or combination of columns in a database table that uniquely identifies each row. No two rows can have the same primary key value.

It serves as a unique identifier, ensuring that each record can be retrieved, updated, or deleted without confusion.

• Must contain unique values.
• Cannot contain NULL values.
• Each table can have only one primary key.

Characteristics of a Good Primary Key

Choosing an appropriate primary key is important for database performance and integrity. Good primary keys have specific characteristics.

• Uniqueness: Each value must be unique across the table.
• Non-nullability: Primary keys cannot be NULL.
• Immutability: Values should not change over time.
• Simplicity: Preferably a single column with simple data types.
• Minimalism: Should contain only necessary data to identify a record.

How Primary Keys Enforce Data Integrity

Primary keys play a critical role in maintaining data integrity by preventing duplicate records and enabling reliable relationships between tables.

They ensure that each record is unique and accessible.

• Uniqueness constraint prevents duplicate entries.
• Primary keys are used as references by foreign keys in related tables.
• They improve query performance by indexing the key columns.

Example: Defining a Primary Key in SQL

Here is a simple example of creating a table with a primary key in SQL.