isnull(email,'') = '' is NOT interpreted by it self as (email is null or email = '')?

Question

as a fact of performance which one is better? Is there a difference between the actual 3 three versions of sql-server (2000 / 2005 / 2008)?

Answer 1

You definitely want to avoid the use of any custom or built-in functions wrapping a column in a filter - it severely limits what the optimizer can do for you in terms of index usage and seekability. You should get in the habit of using equality operators and or/union approaches when possible, as is the case here. The following would be much preferred over a isnull() or coalesce() approach:

where   (
            (t.email is null)
            or
            (t.email = '')
        )

or a union approach as outlined below could work better as well, try it out in your environment to determine which option is best.

A simple example will demonstrate the drastic differences you can see in performance:

use tempdb;
go
if object_id('tempdb..#testTable') > 0
    drop table #testTable;
go
-- Build the dataset
select  top 10000000
        cast(cast(a.name as varchar(100)) + '@' + cast(row_number() over (order by a.object_id) as varchar(15)) + '.com' as varchar(150)) as email, 
        row_number() over (order by a.object_id) as id
into    #testTable
from    sys.columns a
cross join sys.columns b
cross join sys.columns c
go
-- Create some nulls
update  #testTable
set     email = null
where   id % 1000 = 0
go
-- Index
create unique clustered index ixc__dbo_testTable__temp__nc1 on #testTable (email,id) on [default];
go
set statistics io on;
set statistics time on;
go
-- Try with isnull - ~cost of about 44.7 on my machine, ~2900ms to execute, and about 49,200 logical reads
select  *
from    #testTable t
where   isnull(t.email,'') = '';
go
-- Try with 'or' - ~cost of about .049 on my machine, ~643ms to execute, about 31 logical reads
select  *
from    #testTable t
where   (
            (t.email is null)
            or
            (t.email = '')
        );
go
-- Try with union approach - ~cost of about .054 on my machine, ~751ms to execute, ~30 logical reads
select  *
from    #testTable t
where   t.email is null
union all
select  *
from    #testTable t
where   t.email = '';
go
if object_id('tempdb..#testTable') > 0
    drop table #testTable;
go

Answer 2

If you're going to see performance differences, they'll be minute.

I believe the preferred style is

ISNULL(email, '') = ''