Question: List the categories of active businesses in Glendale, AZ.
SQL: SELECT DISTINCT T3.category_name FROM Business_Categories AS T1 INNER JOIN Business AS T2 ON T1.business_id = T2.business_id INNER JOIN Categories AS T3 ON T1.category_id = T3.category_id WHERE T2.active = 'true' AND T2.state = 'AZ' AND T2.city = 'Glendale'