Question: How many object samples in image no.908 are in the class of tip?
SQL: SELECT SUM(CASE WHEN T2.OBJ_CLASS = 'tip' THEN 1 ELSE 0 END) FROM IMG_OBJ AS T1 INNER JOIN OBJ_CLASSES AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T1.IMG_ID = 908