How many object samples in image no.908 are in the class of tip?
object samples in the class of "tip" refer to OBJ_CLASS_ID where OBJ_CLASS = 'tip'; image no.5 refers to IMG_ID = 5;
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