.&.&.& need help about an SQL query &&&

this is my SQL query:

select sum(numOre),sum(numOre*prezzoOra),codLavoro,max(dataImmissione) from
lavlavora
where  (lavlavora.codLavoro = :cLav) and
(lavlavora.dataImmissione >
(select max(dataConteggio) from conteggi where conteggi.codLavoro = :cLav)
or
(select max(dataConteggio) from conteggi where conteggi.codLavoro = :cLav)
is null)

Is there a way to write it without repeating two times the inner "select"?
Unfortunately if max(dataConteggio) is a null value, the comparison with
another date field always gives back a "false"...
I'm not sure whether the data base libraries are optimized about this...
and that could mean that when my tables will have grow longer, my
application could become very very slow.....

thanks for your time