getting values which dont exist in mysql table

Question

I have a set of values(keys), 1,2,3,4,5 and following table:

id key
------
1  2
2  8
3  4

If I need to check which of the given keys are in

Answer 1

This is a bit clunky, but it does work:

select sq.val
from   mytable t
       right join (select 1 as val
                   union 
                   select 2
                   union 
                   select 3
                   union 
                   select 4
                   union 
                   select 5 ) as sq
 on t.col = sq.val
 where t.id is null;

Obligatory SQL Fiddle.