instruction: what is the average cost of the hospital which involves electrolyte administration - phosphate?
response: SELECT AVG(t1.c1) FROM (SELECT SUM(cost.cost) AS c1 FROM cost WHERE cost.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.patientunitstayid IN (SELECT treatment.patientunitstayid FROM treatment WHERE treatment.treatmentname = 'electrolyte administration - phosphate')) GROUP BY cost.patienthealthsystemstayid) AS t1