Null in SQL

Null simply means no value.

In SQL context null can have 3 meaning in a relation.


[1]Not applicable field

e.g. (ID,Name,Spouse Name,Address) ----> (0001,Rambo,null,New York City)
Rambo who is single has no spouse .


[2]Unavailable or Withheld

e.g. (ID,Name,Spouse Name,Address,Tel) ----> (0001,Rambo,null,New York City,Tel)
Rambo want his tel to be confidential hence do not provide it.


[3]Unknown - Don't Know!

e.g. (ID,Name,Spouse Name,Address,Tel, CountryOfBirth) ----> (0001,Rambo,null,New York City,Tel,null)
Rambo does not remember which country he is born from hence the field is left as null.


Thus when null is involve in comparison in SQL three-value logical expression is considered:
T-true,F-false,U-unknown

AND
==============
^|T | F | U
----------------
T|T | F | U
----------------
F| F | F | F
----------------
U|U | F | U
----------------

OR
==============
or|T | F | U
----------------
T|T | T | T
----------------
F| T | F | U
----------------
U|T | U | U
----------------

NOT
==============
! |T | F | U
----------------
= |F | T | U
----------------