3 Value Logic to handle NULL in all kind of Database

3 value logic (TRUE, FALSE, UNKOWN)  in database is contained to give support to NULL, we generally perceived of TRUE or FALSE, but let see how null takes part in Boolean table of OR and AND

In below OR binary logic table, let’s focus on Unknown…

·          Unknown AND True = Unknown

·          Unknown AND False = False

·          Unknown AND Unknown = Unkown

AND	True	False	Unknown
True	True	False	Unknown
False	False	False	False
Unknown	Unknown	False	Unknown

Now let’s take OR and focus on Unknown

Unknown OR True = True

Unknown OR False = Unknown

Unknown OR Unknown = Unknown

OR	True	False	Unknown
True	True	True	True
False	True	False	Unknown
Unknown	True	Unknown	Unknown

Now let’s do a practical, here I am creating #temp1  table which hold 1 to 5 numeric value

select top 5 ROW_NUMBER() over (order by name) as id into #temp1  from sys.objects

In second sql, I am creating another table #temp2 which hold numeric value from 1 to 3 and another value of NULL.

select * into #temp2 from

(

select top 3 row_number() over (order by name) as id from sys.objects

union

select null as id

) a

select * from #temp1;

id

--------------------

1

2

3

4

5

(5 row(s) affected)

select * from #temp2

id

--------------------

NULL

1

2

3

(4 row(s) affected)

Now let run the IN query

select 'wow!', id from #temp1 where id in (select id from #temp2)

which will be internally evaluated as

select 'wow!', id from #temp1 where id = NULL or id = 1 or id = 2 or id = 3

lets revisit OR table then decode the where condition

OR	True	False	Unknown
True	True	True	True
False	True	False	Unknown
Unknown	True	Unknown	Unknown

here if we decode where clause we will find

where  id = NULL or id = 1 or id = 2 or id = 3

for id = 1,

where  Unkown  OR  True OR False OR False, this will result to TRUE (according to the OR table below)

for id =2,

where  Unkown  OR  False OR True  OR False, this will result to TRUE (according to the OR table below)

for id =3,

where  Unkown  OR  False OR False  OR True, this will result to TRUE (according to the OR table below)

for id =4,

where  Unkown  OR  False OR False  OR False, this will result to FASE (according to the OR table below)

for id =5,

where  Unkown  OR  False OR False  OR False, this will result to FASE (according to the OR table below)

So the result of

select 'wow!', id from #temp1 where id in (select id from #temp2)

     id

---- --------------------

wow! 1

wow! 2

wow! 3

(3 row(s) affected)

Now let’s evaluated NOT IN

select 'wow!', id from #temp1 where id not in (select id from #temp2)

it will internally evaluated as

select 'wow!', id from #temp1 where id <> null and id <> 1 and id <> 2 and id <> 3

Now lets revisit AND table and evaluate where condition

AND	True	False	Unknown
True	True	False	Unknown
False	False	False	False
Unknown	Unknown	False	Unknown

here if we decode where clause we will find

where  id <> NULL AND id = 1 AND id = 2 AND id = 3

for id = 1,

where  Unkown  AND  True AND False AND False, this will result to UNKNOWN (according to the AND table below)

for id =2,

where  Unkown  AND  False AND True  AND False, this will result to UNKNOWN (according to the AND table below)

for id =3,

where  Unkown  AND  False AND False  AND True, this will result to UNKNOWN (according to the AND table below)

for id =4,

where  Unkown  AND  False AND False  AND False, this will result to UNKNOWN (according to the AND table below)

for id =5,

where  Unkown  AND  False AND False  AND False, this will result to UNKNOWN (according to the AND table below)

 So the result of

select 'wow!', id from #temp1 where id not in (select id from #temp2)

     id

---- --------------------

(0 row(s) affected)

Enjy coding…SQL J

Post Reference: Vikram Aristocratic Elfin Share