What is the significance of NULL value and why should we avoid permitting null values?

Question

I have a requirement in which a column in the table should have distinct values but it may have multiple null values. How can I achieve that?

Answer 1

Null means no entry has been made. It implies that the value is either unknown or undefined.

We should avoid permitting null values because

Column with NULL values can't have PRIMARY KEY constraints.
Certain calculations can be inaccurate if NULL columns are involved.