What is the category that the most app users belong to?
most app users refers to MAX(COUNT(app_id));
SELECT T.category FROM ( SELECT T1.category, COUNT(T2.app_id) AS num FROM label_categories AS T1 INNER JOIN app_labels AS T2 ON T1.label_id = T2.label_id GROUP BY T1.label_id ) AS T ORDER BY T.num DESC LIMIT 1