Difference between a unique clustered index and a clustered primary key?
why did Microsoft implement "clustered indexes"? Why not just say instead, "It's always clustered on the primary key, and you should define as the primary key whatever you want it to be clustered on"?


To answer this question: The primary key on the table does not have to be the Clustered Index Key. A Clustered Index is the place where SQL Server stores the data of the entire table. The Index itself has what are called "key values." Key Values (1 to many) define how the index is ordered, along with statistics and other useful things. 

In fact, often it's useful to have a Primary Key that is not the Clustered Index Key. For example, some applications create their own keys or GUIDs in order to relate different tables through foreign keys. Using a GUID as a Clustered Index Key is a bad idea though, since GUIDs don't always have an order, unless you create them sequentially. To solve this, you could create a Clustered Index on an identity column, and place the Primary Key on the GUID column.

Your question has many more questions. I think we need to break this down into many questions, since you're asking about a lot of index concepts.