Clustered Index is very important and determine the order in which data is stored in the table. A table without a Clustered Index is called a HEAP. (Try and picture a Heap in your mind and then you can visualize how data is stored in table without a Clustered Index.. Huh!!)
Clustered Index also helps the SQL Engine uniquely identify a row. Thus, if possible, always create a Clustered Index as UNIQUE. If you do not specify the UNIQUE clause while creating your Clustered Index, SQL Server automatically adds a 4 byte uniqueidentifier column to the table to make every row unique.
Thus, specifying a UNIQUE clause also helps save valueable database space.
More information can be found at the following MSDN Page.
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