Among the businesses within the postal code 94117, what is total number of businesses with a high risk category?
businesses with a high risk category refer to business_id where risk_category = 'High Risk';
SELECT COUNT(DISTINCT T2.business_id) FROM violations AS T1 INNER JOIN businesses AS T2 ON T1.business_id = T2.business_id WHERE T2.postal_code = 94117 AND T1.risk_category = 'High Risk'