Clustered index and non-clustered index of SQL Server

Clustered index:The data stored in the table is stored in the order of indexes. the retrieval efficiency is higher than that of normal indexes, but it has a greater impact on the addition, modification, and deletion of data.
Non-clustered index:It does not affect the data storage sequence in the table. The retrieval efficiency is lower than that of clustered indexes, and has little impact on data addition, modification, and deletion.
A table has only one clustered index and multiple non-clustered indexes.

The following table describes the applicability of clustered indexes and non-clustered indexes:

Action Description	Use clustered Index	Use non-clustered Index
Columns are sorted by group.	Ying	Ying
Returns data within a certain range.	Ying	Should not
Returns the exact matched data.	Should not	Ying
One or few different values	Should not	Should not
Different decimal values	Ying	Should not
Different values of large numbers	Should not	Ying
Frequently updated Columns	Should not	Ying
Foreign key column	Ying	Ying
Primary Key Column	Ying	Ying

NOTE: If primary key columns are identified and often used for range queries, clustering indexes are applicable; otherwise, non-clustering indexes are applicable.

Here is a detailed discussion: http://blog.csdn.net/ChinaOffLineSoft/archive/2006/04/11/659725.aspx