What are the captions of all the self-relation relationship prediction classes?
self-relation relationship refers to OBJ1_SAMPLE_ID = OBJ2_SAMPLE_ID and PRED_CLASS; captions of prediction classes refers to PRED_CLASS
SELECT T2.PRED_CLASS FROM IMG_REL AS T1 INNER JOIN pred_classes AS T2 ON T1.PRED_CLASS_ID = T2.PRED_CLASS_ID WHERE T1.OBJ1_SAMPLE_ID = T1.OBJ2_SAMPLE_ID GROUP BY T2.PRED_CLASS