instruction: provide the number of patients whose drug type is main and lab test name is sodium, urine?
response: SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN prescriptions ON demographic.hadm_id = prescriptions.hadm_id INNER JOIN lab ON demographic.hadm_id = lab.hadm_id WHERE prescriptions.drug_type = "MAIN" AND lab.label = "Sodium, Urine"