Yes, we can keep null in unique column if it is nullable. Null is also a unique reconrd in the column. However, you can have only one null record as one null is only unique for that column.
Yes, we can.
The Unique constrain mainly use to restrict duplicate values insertion and ensure that column has a unique value. It will not allow you to enter the duplicate NULL value in the column but allow you to insert a unique(Single) NULL value in the column.
One null value possible, more than that is not possible
Yes
Yes You can .But only one Null value
Yes, But only once . Provided column value is nullable