SQL: Select Keys that doesn't exist in one table

Question

I got a table with a normal setup of auto inc. ids. Some of the rows have been deleted so the ID list could look something like this:

(1, 2, 3, 5, 8,

Answer 1

What about using a left join ; something like this :

select second_table.id
from second_table
    left join first_table on first_table.id = second_table.id
where first_table.is is null

You could also go with a sub-query ; depending on the situation, it might, or might not, be faster, though :

select second_table.id
from second_table
where second_table.id not in (
    select first_table.id
    from first_table
)

Or with a not exists :

select second_table.id
from second_table
where not exists (
    select 1
    from first_table
    where first_table.id = second_table.id
)