What is the difference between clustered and non-clustered indexes?

Clustered and non-clustered indexes are ways of organizing data in a database to speed up searches.

• Clustered Index: determines the physical order of rows in a table. A table can have only one clustered index because data can only be sorted by one key. When searching by a clustered index, data retrieval is faster because they are already ordered.

• Non-Clustered Index: creates a separate structure that contains keys and pointers to the physical location of data. A table can have multiple non-clustered indexes. When searching, the index is accessed first, then the data is retrieved via the pointer.

Example: If you have a users table, a clustered index could be on id (primary key), and a non-clustered index could be on the email field for quick email searches.

Thus, the clustered index affects the physical order of data, while the non-clustered index is an additional structure to speed up searches.