Unique Constraint in SQL

Hi,
 
   Unique Key will Generate Non-Clustured Index.
   UNIQUE Key can have only one NULL  in Sql server (For Oracle : Unique key may have more then one null values because null is equal to another null)

As the name suggests, Unique Key Constraint will allow only unique values in that column. e.g Consider a table Student having Id, Name and Phone columns. Suppose we place unique key constraint on Phone column, then it means that no two rows can have same value in Phone column. In simple words it means that every student will have unique phone.

With this it is quite clear that since null is also a value, there can not be two students who have phone as value with NULL.

Hope this helps.
All the best.

Because then the uniqueness would be lost. Unique means they have to be different in the value. If both are null, then they wouldn't be different.