Why are NULLs not sorted?

You might have a query that you wrote with 'ORDER BY something', and it shows NULLs at the bottom. Then you write it to have 'ORDER BY something DESC', but NULLs don't show up on the top. Although it sounds inconsistent, this is the way SQL standard requires. NULL is not a value, it's a state meaning 'value is unknown'. Since it is unknown, one cannot know whether it is higher or lower than any other value, and Firebird simply puts such records at the end.

If you wish to make your queries 'consistent', you can use NULLS FIRST clause in Firebird 1.5 or higher. If you consider NULLs to be higher than any other value, then use these queries:

select *
from EMPLOYEE
order by phone_ext;

select *
from EMPLOYEE
order by phone_ext desc nulls first;

However, if you consider NULL to be lower than any other value, use the following pair:

select *
from EMPLOYEE
order by phone_ext nulls first;

select *
from EMPLOYEE
order by phone_ext desc;