Question: What is the average difference in the y coordinate of 2 object samples with the relation "parked on" in image no.1?
SQL: SELECT CAST(SUM(T3.Y) AS REAL) / COUNT(CASE WHEN T1.PRED_CLASS = 'parked on' THEN 1 ELSE NULL END) FROM PRED_CLASSES AS T1 INNER JOIN IMG_REL AS T2 ON T1.PRED_CLASS_ID = T2.PRED_CLASS_ID INNER JOIN IMG_OBJ AS T3 ON T2.OBJ1_SAMPLE_ID = T3.OBJ_CLASS_ID WHERE T2.IMG_ID = 1 AND T2.OBJ1_SAMPLE_ID != T2.OBJ2_SAMPLE_ID