SELECT file_id FROM MaTable T1 WHERE category_id = 2 AND EXISTS ( SELECT * FROM MaTable T2 WHERE T2.file_id = T1.file_id AND T2.category_id = 3 )
SELECT T1.file_id FROM MaTable T1 INNER MaTable T2 ON T1.file_id = T2.file_id AND T2.category_id = 3 WHERE T1.category_id = 2