what is the average cost of the total hospital to include a drug called saliva substitute since 3 years ago?
select avg(t1.c1) from ( select sum(cost.cost) as c1 from cost where cost.hadm_id in ( select prescriptions.hadm_id from prescriptions where prescriptions.drug = 'saliva substitute' ) and datetime(cost.chargetime) >= datetime(current_time,'-3 year') group by cost.hadm_id ) as t1