A unique constraint is a type of column constraint within a table that dictates that all values in that column must be unique, even though they may be null.
To ensure that a column is UNIQUE and cannot contain null values, the column must be specified as NOT NULL. Interestingly, these are the two main attributes of a primary key. Defining both attributes in a newly created column should be seriously considered in order to determine the primary key.
A clear constraint is defined at the time a table is created. A clear restriction allows null values. At first this may seem like a contradiction, but a zero is the total absence of a value (not a zero or a space). Therefore it is not possible to say that the value in this null field is ambiguous as nothing is stored in this field.
A null value cannot be compared to an actual value. For example, the Queen of America cannot be compared to the Queen of England because the Queen of America is a zero that doesn't exist.