Modulo syntax/keywords, I think the following query would work in any modern dialect:

    SELECT num FROM 
        (SELECT num FROM nums ORDER BY num DESC LIMIT 2)
    ORDER BY num ASC LIMIT 1;

which is actually nearly identical to the simplest Python solution in terms of the computation that's specified:

    lambda xs: None if len(xs) == 0 else sorted(xs)[-2:][0]

I think the reason the question is tricky for some is because they either:

1) haven't used SQL in a long time and forgot all the syntax, or else

2) don't really "grok" SQL as a model of computation (as opposed to e.g. a data structuring language or something like that).

But both of those seem like good reasons to disqualify someone from a full stack role?