lustered

• Clustered indexes sort and store the data rows in the table or view based on their key values. These are the columns included in the index definition. There can be only one clustered index per table, because the data rows themselves can be sorted in only one order.
  
• The only time the data rows in a table are stored in sorted order is when the table contains a clustered index. When a table has a clustered index, the table is called a clustered table. If a table has no clustered index, its data rows are stored in an unordered structure called a heap.
  

Nonclustered

• Nonclustered indexes have a structure separate from the data rows. A nonclustered index contains the nonclustered index key values and each key value entry has a pointer to the data row that contains the key value.
  
• The pointer from an index row in a nonclustered index to a data row is called a row locator. The structure of the row locator depends on whether the data pages are stored in a heap or a clustered table. For a heap, a row locator is a pointer to the row. For a clustered table, the row locator is the clustered index key.
  
• You can add nonkey columns to the leaf level of the nonclustered index to by-pass existing index key limits, 900 bytes and 16 key columns, and execute fully covered, indexed, queries. For more information, see Create Indexes with Included Columns
  
  

  Index Types

  There are two main index types; Clustered index and Non-Clustered index.
  
  A clustered index alters the way that the rows are stored. When you create a clustered index on a column (or a number of columns), SQL server sorts the table’s rows by that column(s). It is like a dictionary, where all words are sorted in alphabetical order in the entire book. Since it alters the physical storage of the table, only one clustered index can be created per table. In the above example the entire rows are sorted by computer_id since a clustered index on computer_id column has been created.

  CREATE CLUSTERED INDEX [IX_CLUSTERED_COMPUTER_ID] 
  ON [dbo].[nics] ([computer_id] ASC)

  A non-clustered index, on the other hand, does not alter the way the rows are stored in the table. It creates a completely different object within the table that contains the column(s) selected for indexing and a pointer back to the table’s rows containing the data. It is like an index in the last pages of a book, where keywords are sorted and contain the page number to the material of the book for faster reference. A non-clustered index on the computer_id in the previous example would look like the table below:

  CREATE NONCLUSTERED INDEX [IX_NONCLUSTERED_COMPUTER_ID] 
  ON [dbo].[nics] ([computer_id] ASC)

  Here is the common misconception prevailing in the industry.

  Primary Key has to be Clustered Index. 

  In reality the statement should be corrected as follows:

  Primary Key can be Clustered or Non-clustered but it is a common best practice to create a Primary Key as Clustered Index. 

  
  

  
  

  Scenario 1 : Primary Key will default to Clustered Index

  In this case we will create only Primary Key and when we check the kind of index created on the table we will notice that it has created clustered index automatically over it.

  -- Case 1 Primary Key Defaults to Clustered Index
USE TempDB
GO
-- Create table
CREATE TABLE TestTable
(ID INT NOT NULL PRIMARY KEY,
Col1 INT NOT NULL)
GO
-- Check Indexes
SELECT OBJECT_NAME(OBJECT_ID) TableObject,
[name] IndexName,
[Type_Desc]
FROM sys.indexes
WHERE OBJECT_NAME(OBJECT_ID) = 'TestTable'
GO
-- Clean up
DROP TABLE TestTable
GO