unique vs key in sql code example

Example 1: sql distinct vs unique

Unique: helps to ensure that all the values
in a column are different
Distinct: helps to remove all the duplicate
records when retrieving the records from a table.

Example 2: difference between unique vs primary key in sql

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.