mysql如何去除两个字段数据相同的记录

思路：

1.先查询一样的数据

select * from table where num1 in(select num2 from table)

2.进行删除

delete table where id in(select id from table where num1 in(select num2 from table))

假设表名为test：

select * from (select *, concat(name,code) as __f from test order by date desc) __t group by __f

select name,grade,testGrade,count(*) from t_student group by (name,grade,testGrade) having count(*)>1 where grade is null

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