Keys (1/26)
What is a key in MySQL and why is it used?
    Understanding Keys in MySQL and Their Purpose

    In MySQL, a key is a column or a set of columns that helps identify, organize, and optimize access to data within a table. Keys are essential for ensuring data integrity and improving query performance.

    1. Why Keys Are Used
    • To uniquely identify rows (e.g., PRIMARY KEY).
    • To enforce relationships between tables (e.g., FOREIGN KEY).
    • To speed up data retrieval using indexes (any key automatically becomes an index).
    • To prevent duplicate values when required (e.g., UNIQUE KEY).
    2. Types of Keys in MySQL
    • **Primary Key** – Uniquely identifies each row and cannot contain NULL.
    • **Unique Key** – Ensures all values are distinct but can contain NULLs.
    • **Foreign Key** – Enforces referential integrity between related tables.
    • **Index Key** – Used to improve search performance on selected columns.
    • **Composite Key** – A key made of more than one column.
    Example: Defining Keys

    Keys are central to relational database design—they ensure correctness of data and significantly improve query efficiency by allowing MySQL to quickly locate rows instead of scanning entire tables.