instruction: what is the average cost of the total hospital to include a drug called saliva substitute since 3 years ago?
response: 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