Run the following code

	declare @t table(i int unique)
	insert into @t 
	select 1 union all
	select null union all
	select null 

You will get the following error

Msg 2627, Level 14, State 1, Line 3
Violation of UNIQUE KEY constraint 'UQ__#A989971__3BD019979D26A3BB'. Cannot insert duplicate key in object 'dbo.@t'. The duplicate key value is ().
The statement has been terminated.

But other RDBMSs like ORACLE, MySQL, etc will allow multiple NULL values on a column defined as Unique key. It is because that NULL values cannot be equal to each other

Why does SQL Server allow only one NULL value on a column that has unique constraint?