WHAT DOES A UNIQUE KEY MEAN IN SQL code example

Example 1: unique vs primary key

What is the difference between UNIQUE and PRIMARY KEY constraints? 
There should be only one PRIMARY KEY 
in a table whereas there can be
any number of UNIQUE Keys.
PRIMARY KEY doesn’t allow NULL
values whereas Unique key allows NULL values.

Example 2: unique key

Unique Key:
Only unique value and also can contain NULL