Why a table can have only single clustered index ?

Question

My table is too big and now i realize that i need to add few columns in that, but when i tried to do this, my alter script took more then 4 hours and database performance also got degraded, so i canceled the query, can anyone suggest better way of doing that

Answer 1

A clustered index sorts and stores the data rows in the table based on the index key values. Therefore only one clustered index can be created on each table because the data rows themselves can only be sorted in one order.