根据多个字段查询重复数据：SELECT A,B,C FROM TABLE WHERE CONDITION GROUP BY A,B,C HAVING COUNT(*)>1即可，但是现在的需求是：

最终查询的字段多于分组字段，且同一字段的空值也视为重复。在网上查询了很多资料，也询问了同事最后尝试出如下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（）函数中的后一个值是自己设置的，但设置的值的类型要与前一个值的类型相同。

如果要处理相同条件下查询出的数据，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))

这里涉及到IN与EXISTS,NOT IN与NOT EXISTS的区别，有兴趣的同学可以查一查。

虽然能实现查重及去重功能，但是在大数据量时模型会运行特别慢，和数据库也有一定关系。