Question

I was surprised to know that SQL Server (I tested the 2008 R2 edition, but this seems to work in earlier versions too) allows inserting data into tables which do not have a primary key (and hence a clustered index) defined.

How is it possible and how the data are stored physically for these tables?

Answer 1

Helpful link here:

http://www.sqlmag.com/blog/sql-server-questions-answered-28/sql-server/what-happens-if-i-drop-a-clustered-index-137109

Q: I’ve heard that the clustered index is “the data,” but I don’t
fully understand what that means. If I drop a clustered index, will I
lose the data?

A: I get asked this question a lot, and I find that index structures
tend to confuse people; indexes seem mysterious and, as a result, are
unintentionally thought of as very complicated. A table can be stored
internally with or without a clustered index. If a table doesn’t have
a clustered index, it’s called a heap. If the table has a clustered
index, it’s often referred to as a clustered table. When a clustered
index is created, SQL Server will temporarily duplicate and sort the
data from the heap into the clustered index key order (because the key
defines the ordering of the data) and remove the original pages
associated with the heap. From this point forward, SQL Server will
maintain order logically through a doubly-linked list and a B+ tree
that’s used to navigate to specific points within the data.