根據多個欄位查詢重複資料：SELECT A,B,C FROM TABLE WHERE CONDITION GROUP BY A,B,C HAVING COUNT(*)>1 即可，但是現在的需求是：

最終查詢的欄位多於分組欄位，且同一欄位的空值也視為重複。在網上查詢了很多資料，也詢問了同事最後嘗試出如下sql：

SELECT A,B,C,D,E FROM TABLE A WHERE EXISTS(SELECT A,B,C FROM TABLE B WHERE CONDITION AND COALESCE(A.A,'0')=COALESCE(B.A,'0') AND COALESCE(A.B,'0')=COALESCE(B.B,'0') AND COALESCE(A.C,'0')=COALESCE(B.C,'0') GROUP BY A,B,C HAVING COUNT(*)>1);

注意：上述sql中coalesce（）函式中的後一個值是自己設定的，但設定的值的型別要與前一個值的型別相同。

如果要處理相同條件下查詢出的資料，可使用如下sql：

DELETE FROM TABLE WHERE ID NOT IN(SELECT ID FROM

(SELECT MIN(ID) ID,A,B,C FROM TABLE WHERE CONDITION GROUP BY A,B,C HAVING COUNT(*)>1) C) 

AND ID IN(SELECT ID FROM TABLE A WHERE EXISTS 

(SELECT A,B,C FROM TABLE B WHERE CONDITION AND COALESCE(A.A,'0')=COALESCE(B.A,'0') AND COALESCE(A.B,'0')=COALESCE(B.B,'0') AND COALESCE(A.C,'0')=COALESCE(B.C,'0') GROUP BY A,B,C HAVING COUNT(*)>1))

這裡涉及到IN 與EXISTS,NOT IN與NOT EXISTS的區別，有興趣的同學可以查一查。

雖然能實現查重及去重功能，但是在大資料量時模型會執行特別慢，和資料庫也有一定關係。