Premise Description: The database is used in mysql.
Database table:
Title:
Delete name the same line, and the line is reserved for the smallest line.
Ideas:
1. Find a different number in addition to, other incomplete information the same number.
Keywords: group by ...... having: grouping query, my understanding of this keyword is: Find the column between different rows of the same or a few, a few require the same separated by commas. A few lines of the value of the query if required filter condition, you need to filter by having.
sql statement is as follows:
the SELECT from the above mentioned id t_test20191023 Group by name
2. delete information from the summary table in the query is not the first step to get the number of data
delete from t_test20191023 where id not in ( select id from t_test20191023 group by name order by id)
I write to you, sql statement written by the seemingly perfect, but when running mysql, this error occurs:
The cause of the error is: update the table at the same time this query tables, queries the table at the same time went to update the table, it can be understood as a deadlock, so the emergence of this issue.
解决方法：
在第一步中查询最小的id时，不从主表中去查找，而是根据需要的字段从构建一个第三个表，从第三个表中去获取数据。
select id from (select id from t_test20191023 group by name order by id)
报错
[Err] 1248 - Every derived table must have its own alias
必须给表起个别名
select id from (select id from t_test20191023 group by name order by id) b
然后删除
这样也是可以的：delete from t_test20191023 where id not in (select minid from (select min(id) as minid from t_test20191023 group by name order by id) b)