Obviously the article is posted by somebody who has no clue about database fundamental theory....The writer just made up the *Four Rules* to make it sound scientific but in fact, most of what's written is just a bunch of crappola...
NULLS are the poorest possible way to handle missing data on any direct image system(SQL Server, ORACLE, DB2), which is why they should never be used in the first place. So the message which learn to use them is basically the same as saying: NULLS will mess up your database but here is how to make them less harmful. I say don't use them...
Some proofs of the absurdity of using NULLS in SQL Server.
First, let's create a simple table and fill it with data
create table table1(field1 int, field2 int)
go
insert table1
select 1, 1
insert table1
select 2, 1
insert table1
select 3, 2
insert table1
select 4, NULL
The table has now
field1 field2
11
21
32
4NULL
Proof1:
run this
now run....
select sum(field1 + field2) from table1
--> it returns 10
then run...
select(field1) + sum(field2) from table1
which should produce the same righ. Wrong!!
--> it return 14 !
So use of NULLS will mess up your sum results...
Proof2:
on the above table run this...
select * table1 where field1 = field2
according to the data, it returns all matching records between field1 and field2...The query returns
field1 field2
11
Now run
select * from table1 where field1 field2
to return non matching records in column field1 and column of field2...Where on woulmd expect the three last records, the system returns
field1 field2
21
32
As you can see the system does consider that 4 = NULL a total onsense. 3VL will somebody say but 3VL does not apply in relational modeling. Only 2VL logic applies in Relation Model else it is nothing but relational.
Proof3 (the proof that 3VL logic is nothing but bullshit):
Based on the above Proofs how do we get the entire table?
well basically,
select * from table1
is equivalent to
select * from table1 where field1 = field2
union
select * from table1 where field1 field2
union
select * from table1 where field2 is null
which total nonsense...and adds unecessary complexity
select * from table1
should indeed equal to
select * from table1 where field1 = field2
union
select * from table1 where field1 field2
why use a logic that breaks pretty much all prequisites that make a system relational, get exposed to false results unless putting tons of IS NULL/IS NOT NULL conditions that will degrade performance by additional index scans when one can do without them.