总结：
1、查询时用 not in 效率极其低下，因此结合left join改为in查询，效率很快
原语句:
select * from my_test_table where id not in (select b.id as id from ( SELECT MAX(a.`ModifyAt`)ModifyAt,a.userid FROM my_test_table a GROUP BY a.UserId,a.SeriesId,a.CourseId) c INNER JOIN `my_test_table` b ON c.userid=b.userid AND c.ModifyAt=b.ModifyAt)
优化后：
select * from (select u.id as id from my_test_table u left join (select b.id as id from ( SELECT MAX(a.`ModifyAt`)ModifyAt,a.userid FROM my_test_table a GROUP BY a.UserId,a.SeriesId,a.CourseId) c INNER JOIN `my_test_table` b ON c.userid=b.userid AND c.ModifyAt=b.ModifyAt) d on d.id = u.id where d.Id is null) as e
2、查询出后，需要用 delete where in 来删除，但是delete where in 语句效率极其低下。
原语句：
delete from my_test_table where id in (select * from (select u.id as id from my_test_table u left join (select b.id as id from ( SELECT MAX(a.`ModifyAt`)ModifyAt,a.userid FROM my_test_table a GROUP BY a.UserId,a.SeriesId,a.CourseId) c INNER JOIN `my_test_table` b ON c.userid=b.userid AND c.ModifyAt=b.ModifyAt) d on d.id = u.id where d.Id is null) as e)
因此改为where语句，关联表删除数据 优化后：
delete deluser from my_test_table deluser,(select * from (select u.id as id from my_test_table u left join (select b.id as id from ( SELECT MAX(a.`ModifyAt`)ModifyAt,a.userid FROM my_test_table a GROUP BY a.UserId,a.SeriesId,a.CourseId) c INNER JOIN `my_test_table` b ON c.userid=b.userid AND c.ModifyAt=b.ModifyAt) d on d.id = u.id where d.Id is null) as e) f where deluser.id = f.id;
0.36秒执行完成。
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