A Not Null constraint is a rule that prevents a column from having a NULL value. A NULL value means that there is no known value for that field. For example, if you have a table called person with a column called name, and you want to make sure that every person has a name, you can use a Not Null constraint on the name column.

A Null constraint is the opposite of a Not Null constraint. It allows a column to have a NULL value. This means that the field can be left empty or unknown. For example, if you have a table called person with a column called phone$\_$number, and you want to allow some people to not have a phone number, you can use a Null constraint on the phone$\_$number column.

Is NULL value a blank in a field. Is there a way to check if the field has NULL or not?