关于mysql中删除重复记录，并保留重复数据中的一条数据的SQL语句理解

正好想写一条删除重复语句并保留一条数据的SQL，网上查了一部分资料写的很详细，但还是在这里写下自己的理解，以遍后续学习 。如下：

表字段和数据：

SQL语句： 

DELETE FROM `user` WHERE id NOT IN(SELECT * FROM(SELECT id FROM `user` GROUP BY username)AS b)  临时表

理解：

先从里面的SQL开始看

1、SELECT id FROM `user` GROUP BY username  根据名字分组查询出每组的ID。

2、SELECT * FROM(SELECT id FROM `user` GROUP BY username) AS b  这句话中有2个疑问点，

  第一、为什么要套这样一个select？因为 更新数据时使用了查询，而查询的数据又做更新的条件，mysql不支持这种方式

    如果不套上这个select查询，那么将会报1093 -  You can't specify target table 'user' for update in FROM clause错误。

第二、这句话中一定要取别名，不然会报1248 - Every derived table must have its own alias 错误

3、结合上面的分析来看一下整个的SQL语句理解，先将分组的ID查出来，然后删除USER表中ID 不在分组ID中的数据，那么就实现效果了。

delete from 表名 where  ID not in (select * from (select  id from 表名 group by 分组的列名)   别名)

效果如下：

print('MySQL数据库正在连接，请稍等。。。。。。')

db= pymysql.connect('27.255.79.21','user','*******','db_qishu',charset="utf8")

#db = pymysql.connect('127.0.0.1', 'root', 'root', 'db_qishu', charset="utf8")

cursor= db.cursor()

print('MySQL数据库已经连接！！！！！！')

#sql = "SELECT Novel_url,count(Novel_url) as count FROM qishu_books_sort08 GROUP BY Novel_url HAVING count(Novel_url)>1 ORDER BY count ASC;"

#sql = "select Novel_url,count(*) as count from qishu_books_sort08 group by Novel_url having count>1;"  #120704

#sql = "select Novel_ID,Novel_title from qishu_books_sort08 where Novel_url in (select Novel_url from qishu_books_sort08 group by Novel_url having count(Novel_url)>1) ORDER BY Novel_ID ASC;"

#sql = "SELECT Novel_url as count FROM qishu_books_sort011 GROUP BY Novel_url ORDER BY count ASC;"

#sql = "select Novel_ID from qishu_books_sort08 group by Novel_url having count(Novel_url) > 1;"

sql= "SELECT Novel_ID FROM `qishu_books_sort08` GROUP BY Novel_url ORDER BY Novel_ID ASC;"

cursor.execute(sql)

TXT_list= cursor.fetchall()

print(TXT_list)

print(len(TXT_list))

---

mysql可以根据字段进行排序

其中，DESC表示降序，ASC表示升序

order by 字段名  DESC;按照字段名降序排序

order by 字段名 ASC;按照字段名升序排序