What's the difference between clustered and non-clustered indexes? When do you use them for assigning UNIQUE constraints? The answer posted in Difference between clustered and non-clustered indexes (16 May 2001) did not explain about non-clustered indexes. More explanation with a simple example is requested.
This question is really beyond the scope of standard SQL. Whether clustered indexes are even possible depends on which database system you are using. My background is in SQL and database design, not database administration, which is the skill set that includes database performance, tuning, and issues such as clustered and non-clustered indexes.
However, I did want to post a response anyway, simply to encourage you to keep asking the question
In addition, you can also try searching. If you search for "clustered and non-clustered indexes" at SearchOracle.com, you will find a number of tips and articles (including the one by me mentioned above), as well as articles at other sites, such as
Tips on Optimizing SQL Server Clustered Indexes and Tips on Optimizing Non-Clustered SQL Server Indexes at sql-server-performance.com
SQL Server Index Tuning at about.com