SQL Server might have bug when dealing with 'null' value

SQL Server might have bug when dealing with 'null' valueDeclare @N as intset @N=NULLSelect len(isnull(@N,''))Select len(isnull(NULL,''))It should give output ‘0’ for both query. Check by yourself.Results coming as:
1
0

That is because you cant set an int to empty string. it would be 0 which has a length of 1. The one where you dont declare a type is defaulted to char I believe... Try below..Declare @N as int
set
@N=NULLSelect
len(isnull(@N,''))Select
len(isnull(NULL,''))Select
isnull(@N,'')

Let's not forget that len(null) returns null too. And perhaps OP does not fully understand the point of the ISNULL() function.
Do not mistake predictable behaviour of SQL Server for a bug. What you need to do yourself, as a DBA or a programmer, is to anticipate correctly how SQL Server handles nulls.

If not a bug, then that is a problem
I have a comeplecated query when I have to check then lenth of an integer and padding 0 to make same length,
Select len(isnull(intFieldName,'')) -funtion giving 0-9 same value as when null.

You're mixing character and integer data - don't!
What about LEN(ISNULL(intFieldName, 0) - or try a CASE expression:

SELECT CASE WHEN intFieldName IS NULL THEN ... ELSE ... END

But what you are trying to do is make a int be an empty string. It cant. it is not a string that can be empty. The problem is your understaning of this concept it seems. a isnull for a int should be isnull(intfield,0) not isnull(intfield,'').

About IsNull From Books Online...
ISNULL
Replaces NULL with the specified replacement value.
Syntax
ISNULL ( check_expression , replacement_value )
Arguments
check_expression
Is the expression to be checked for NULL. check_expression can be of any type.
replacement_value
Is the expression to be returned if check_expression is NULL. replacement_value must have the same type as check_expresssion.
Return Types
Returns the same type as check_expression[]

can you imagine that buzzle:
Declare @N as int
set @N=NULL
Select len(isnull(@N,''))
Select len(isnull(NULL,''))

it return
1
0
--------------------------
Declare @N as datetime
set @N=NULL
Select len(isnull(@N,''))
Select len(isnull(NULL,''))
it return
19 [] []
0
------------------------
Declare @N as char
set @N=NULL
Select len(isnull(@N,''))
Select len(isnull(NULL,''))

it return
0
0
-----------------------
Declare @N as sysname
set @N=NULL
Select len(isnull(@N,''))
Select len(isnull(NULL,''))
it return
0 []
0

.... and so on
so it is not a bug , but it is due to the internal storage of NULL for the different datatype