SQL Clustered Index

A primary key is usually a unique clustered index. A primary key cannot be null.

A unique clustered index, if not a primary key, can be null.

A clustered index determines the physical order of rows in a table.

There can be at most one clustered index on a table. Non-clustered index is built upon clustered index, and can be created only when a clustered index already exists on the table.

A unique key is non-clustered by default, unless you specify it to be clustered.

[1] Creating Clustered Indexes
[2] Using Clustered Indexes
[3] CREATE INDEX (Transact-SQL)