Comparing with null by converting value to a constant with isnull

Question

One of our programmers tends to use isnull in MS SQL to compare with NULLs. That is instead of writing Where ColumnName Is Null he writes Where IsNull(ColumnName,0)=0

I think that the optimizer will convert the latter into the former anyway, but if it does not - is there a way to prove that the latter is less effective, since it 1.Compares with null, 2.Converts to integer, 3.Compares 2 integers instead of just comparing with null.

Both ways are really fast for me to be able to use the execution plans (and also I think, that the optimizer plays its part). Is there a way to prove him that just comparing with Null without IsNull is more effective (unless it's not).

Answer 1

Another obvious issue is the ISNULL precludes the use of indexes.

Run this setup:

create table T1 (
    ID int not null primary key,
    Column1 int null
)
go
create index IX_T1 on T1(Column1)
go
declare @i int
set @i = 10000
while @i > 0
begin
    insert into T1 (ID,Column1) values (@i,CASE WHEN @i%1000=0 THEN NULL ELSE @i%1000 END)
    set @i = @i - 1
end
go

Then turn on execution plans and run the following:

select * from T1 where Column1 is null
select * from T1 where ISNULL(Column1,0)=0

The first uses an index seek (using IX_T1) and is quite efficient. The second uses an index scan on the clustered index - it has to look at every row in the table.

On my machine, the second query took 90% of the time, the first 10%.

Answer 2

IsNull is not well used if you are using it in the where clause and comparing it to 0, the use of isnull is to replace the value null

http://msdn.microsoft.com/en-us/library/ms184325.aspx

For example:

SELECT Description, DiscountPct, MinQty, ISNULL(MaxQty, 0.00) AS 'Max Quantity'
FROM Sales.SpecialOffer;