TRC: Select an ID if it appears on all measurements

I have these two "tables": Station (station_id, city) // Stations details Rainfall (station_id, date, amount) // Rainfall measurements With **Tuple Relational Calculus** (TRC), I need to get the IDs of all stations that have related measurements on every date in which *any* measurement was taken. So with the following tables I'm supposed to select 5 because this station has measurements on every date that has a measurement. Station Rainfall ------- -------- --station_id-----city-- --station_id-----date-----amount-- 5 LA 5 01/01 4 7 NY 7 02/02 8 5 02/02 3 I've tried this but I'm not quite sure if this would work if there are no measurements at all (in this case I'm supposed to select **all** station IDs): { t | ∃s ∈ Station (t[station_id] = s[station_id] ∧ ∀r ∈ Rainfall (∃q ∈ Rainfall (r[date] = q[date] ∧ s[station_id] = q[station_id])) ) } I need your help in understanding if what I did is correct.