将数据进行排序，取前几条

 SELECT A.DIAG_ID,A.DIAG_NAME,NVL(B.FIT_PCT,0) FIT_PCT,B.FIT_TYPE FROM T_DIC_DIAG A
 left join  (SELECT T.* ,ROW_NUMBER() OVER(PARTITION BY DIAG_ID ORDER BY FIT_PCT DESC) rn from t_diag_icd t) b on b.rn=1 and a.diag_id=b.diag_id