Mysql：如何删除所有违反UNIQUE约束的重复项

Question

我想向表添加UNIQUE索引，如下所示：

ALTER TABLE `mytable` ADD UNIQUE `myunique_name`(`first`, `second`, `third`);

Mysql响应：

Duplicate entry '1-2-3' for key 'myunique_name'

我肯定知道，这种组合只是违反约束的千分之一。

在这种特殊情况下，我可以确定三个指定列中包含相同值的所有行在其他相关字段中也包含相同的数据（主索引当然不同，但是无关紧要），因此所有重复项都可以被删除。

有没有一种方法可以删除所有重复的条目但保留一个 （不管保留哪个主键都没有关系）以便可以添加唯一索引？

Answer 1

CREATE TEMPORARY TABLE IF NOT EXISTS MyTable engine=memory 
select 1 as id, 1 col1,1 col2,1 col3
union all
select 2 as id, 2 col1,2 col2,2 col3
union all
select 3 as id, 3 col1,3 col2,3 col3
union all
select 4 as id, 4 col1,4 col2,4 col3
union all
select 5 as id, 1 col1,1 col2,1 col3
union all
select 6 as id, 2 col1,2 col2,2 col3


CREATE TEMPORARY TABLE IF NOT EXISTS MyDuplicateTableWithCount engine=memory 
select col1  , col2  , col3, count(*) Count_1
from MyTable
group by col1  , col2  , col3
having count(*)>1


  select a.* from  MyTable a
    inner join 
    (select col1  , col2  , col3
    from MyDuplicateTableWithCount
     ) b
     on a.col1 =b.col1 and a.col2 =b.col2 and  a.col3 =b.col3
    order by a.id

获得重复ID后，编写删除查询，指定重复ID为

delete from myTable where id in (5,6)

---

还可以从上方使用myTable使用以下查询

CREATE TEMPORARY TABLE IF NOT EXISTS MyTable2 engine=memory 
 SELECT MIN(id) as id, Col1, Col2, Col3 
   FROM MyTable 
   GROUP BY Col1, Col2, Col3

DELETE a FROM MyTable as a
LEFT  JOIN (
   SELECT * from MyTable2
) as b ON
   b.id = a.id
WHERE
   b.id IS NULL