[T-SQL] Difference between Primary Key and Unique Key

Primary Key:

1. Enforce uniqueness of the column on which they are defined.
2. By default, creates a clustered index on the column.
3. Doesn't allow NULL value.
4. A table can have only one primary key.

Unique Key:

1. Enforce uniqueness of the column on which they are defined.
2. By default, creates a non-clustered index on the column.
3. Allows one NULL value.
4. A table can have multiple unique keys.