Null means "undefined" or "unspecified" or "empty"
The value of null is undefined although it is usually treated as empty.
Null is the default value of all variables. It is the variable's value before it has been given a value.
Null can be assigned to any variable.
Null can behave like zero -
Null is always treated as zero in numeric operations
Null is always treated as zero in boolean operations
I like to think that contemplating null is at the centre of the understanding of data.
Every attribute that will have existed was null before being given a value. Null is the value of all attributes that will exist