Core Concept Mastery • DBMS Module
Think of a Textbook.
Clustered Index: This IS the book. The pages are physically glued in order (Page 1, 2, 3...).
Because you can only physically sort the pages in ONE way, you can have only ONE Clustered Index per table.
Non-Clustered Index: This is the "Index" section at the very back of the book.
It lists topics alphabetically: "Atom... Page 50".
The topics are sorted A-Z, but the actual pages are not.
It's a look-up list. You find the topic, get the pointer (Page 50), and then go to the actual page.
Without an index, the database has to scan every single row (Full Table Scan) to find "Alice." With an index, it jumps straight to "A".
Clustered = The dictionary itself (words are sorted A-Z). Non-Clustered = The index page in a Textbook (Topic Name -> Page 24).
"How many clustered indexes can a table have?"
Only One (because data can be physically sorted in only one way).