unique key in sql server code example

Example 1: set column as unique in sql server

ALTER TABLE Persons
ADD UNIQUE (ID);

Example 2: what is unique key in sql

Unique Key:
Only unique value and also can contain NULL

Example 3: primary key and unique 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.

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