Difference between Primary Key and Unique Key in SQL Server

Difference between Primary key and Unique Key
(Primary key vs Unique key)

• Both primary key and unique enforce uniqueness of the column on which they are defined.

• But, by default, primary key creates a clustered index on the column, where as unique key creates a non- clustered index by default.

• Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.

• Also, there can only be one primary key, while there can be many Unique keys