MySQL删除重复行保留一个[关闭]

Question

我的桌子是这样的：

Event_id	Species
1	Dog
1	Horse
2	Dog
3	Cat
4	Fish
4	Bird
5	Cat

我不想保留任何具有重复 event_id 的行，因为我无法确定事件的物种类型。如何在mysql中删除表的两行？我没有每行的唯一 ID。

输出应如下所示：

Event_id	Species
2	Dog
3	Cat
5	Cat

提前致谢！

Answer 1

这是我在 MySQL 8.0 上测试的一个解决方案（使用with 需要）：

mysql> create table mytable (event_id int, species varchar(20));
mysql> insert into mytable (Event_id,Species) values (1,'Dog'), (1,'Horse'), 
    (2,'Dog'), (3,'Cat'), (4,'Fish'), (4,'Bird'), (5,'Cat');

mysql> with cte as (select event_id from mytable group by event_id having count(*)>1) 
    delete mytable from mytable join cte using (event_id);

mysql> select * from mytable;
+----------+---------+
| event_id | species |
+----------+---------+
|        2 | Dog     |
|        3 | Cat     |
|        5 | Cat     |
+----------+---------+

Answer 2

一个简单的方法是：

delete t1 
from my_tbl as t1  
inner join  (  select event_id
               from my_tbl
               group by event_id
               having count(*) >1 
              )as t2 
on t1.event_id=t2.event_id;

演示：https://www.db-fiddle.com/f/7yUJcuMJPncBBnrExKbzYz/155

或者使用sybquery：

delete from my_tbl  
where event_id not in ( select t1.event_id from (select event_id
                                              from my_tbl
                                              group by event_id
                                              having count(*) =1) as t1
                      ) ;

演示：https://www.db-fiddle.com/f/7yUJcuMJPncBBnrExKbzYz/151

以下查询返回仅存在一次的 event_id。所以你可以删除条件event_id not in。

select event_id
from my_tbl
group by event_id
having count(*) =1