Understanding SQL Server Indexing for Beginners

If you have a table with millions of rows, searching it without an index is like trying to find a specific word in a textbook without a glossary. You have to read every single page (a Table Scan).

Clustered Indexes

A Clustered Index dictates the physical order of the data on the hard drive. A table can only have ONE clustered index (usually the Primary Key). If a phonebook is ordered by Last Name, that is its clustered index.

Non-Clustered Indexes

A Non-Clustered index is a separate structure (like the index at the back of the book). You can have many of them. If you frequently search your database by "EmailAddress", you should create a non-clustered index on that column.

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