parameter to check is null or is not null in sql server 2008?

I have one table Employee

       Eno     ename   image

       1        aaa     a.jpg
       2        bbb     b.jpg
       3        ccc     null
       4        ddd     nul开发者_Python百科l

I pass the parameter to the query

   declare @a varchar(10)

   select * from employee where ?

? if i pass image is not null mean i want all employee details who have image other wise

i want image is null employee details who not have image.

select * 
from employee 
where (@a is null and image is null)
    or (@a is not null and image is not null )

Not 100% sure but I think you want

WHERE     (@a is null AND image is NULL)
   OR (@a is not null AND image is not NULL)

This is short and sweet but not SARGable so it won't perform well:

select *
    from employee
    where coalesce(image,'') = coalesce(@a,'')

For better performance, I'd go with:

if @a is not null
    select *
        from employee
        where image = @a
else
    select *
        from employee
        where image is null