instruction: what is the number of patients on main type drug prescription who have diagnoses of myalgia and myositis, unspecified?
response: SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN diagnoses ON demographic.hadm_id = diagnoses.hadm_id INNER JOIN prescriptions ON demographic.hadm_id = prescriptions.hadm_id WHERE diagnoses.long_title = "Myalgia and myositis, unspecified" AND prescriptions.drug_type = "MAIN"