SELECT that returns list of values not occurring in any row

Question

Query:

select id from users where id in (1,2,3,4,5)

If the users table contains ids 1, 2, 3, this would return 1, 2, and 3. I want a query

Answer 1

select missing.id
  from
(select ELT(@indx, 1,2,3,4,5) as id, @indx:=@indx+1
  from (select @indx:=1) init,
    users
  where ELT(@indx, 1,2,3,4,5) is not null
) missing
  left join users u using(id)
where u.id is null;

What you have in here is:

• ELT together with the variable @indx allows you to 'transpose' the list into a column.
• (select @indx:=1) is needed to initialize indx to 1
• and users table in the inner select is needed so that MySQL has something to iterate on (so the size of your list cannot exceed the number of the rows in users table, if that's the case that you can use any other table that is big enough instead of inner users, again, table itself does not matter it's just to have something to iterate on, so only its size that matters).
• ELT(@indx, 1,2,3,4,5) is not null condition in the nested select is to stop iteration once you are at the index exceeding your list size.

The rest is simple - left join and check for null.