What is the device model used by the most female users over 30?
female users refers to gender = 'F'; most female users refers to MAX(COUNT(gender = 'F')); over 30 refers to age > 30;
SELECT T.device_model FROM ( SELECT T2.device_model, COUNT(T2.device_model) AS num FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T1.age > 30 AND T1.gender = 'F' GROUP BY T2.device_model ) AS T ORDER BY T.num DESC LIMIT 1