How many images have at least one pair of object samples with the relation "parked on"?
How many images have at least one pair of object samples with the relation "parked on" refers to count(IMG_ID) where OBJ1_SAMPLE_ID ! = OBJ2_SAMPLE_ID and PRED_CLASS = 'parked on'
SELECT SUM(CASE WHEN T1.PRED_CLASS = 'parked on' THEN 1 ELSE 0 END) FROM PRED_CLASSES AS T1 INNER JOIN IMG_REL AS T2 ON T1.PRED_CLASS_ID = T2.PRED_CLASS_ID WHERE T2.OBJ1_SAMPLE_ID != T2.OBJ2_SAMPLE_ID