Tell the number of fights landed earlier on Miami Airport on 2018/8/12.
landed on refers to DEST; landed earlier refers to ARR_DELAY < 0; Miami Airport refers to DEST = 'MIA'; on 2018/8/12 refers to FL_DATE = '2018/8/12';
SELECT COUNT(*) FROM Airports AS T1 INNER JOIN Airlines AS T2 ON T1.Code = T2.DEST WHERE T2.FL_DATE = '2018/8/12' AND T2.DEST = 'MIA' AND T2.ARR_DELAY < 0