I've following SQL statement:

select L_ID, L_NUMBER, lt.LT_TIMESTAMP as START_TIME
from L_TABLE
left join L_TABLE_SLAVE lt on lt.LT_L_ID = L_TABLE.L_ID and lt.LT_EVENT_ID = 1
where L_TABLE.L_ID > 177700

I'd like to limit search resutl on to rows where START_TIME is null.
However my first idea ".. AND START_TIME IS NULL" doesn't work, but
second guess ".. AND lt.LT_TIMESTAMP IS NULL" works.

Is it possible to use use "START_TIME" in query at all?

I'm using Firebird 2.0.4 with this one.