instruction: how many patients prescribed with base drug type have been diagnosed with von willebrand's disease?
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 = "Von Willebrand's disease" AND prescriptions.drug_type = "BASE"